[Feat] (MG_Impl, MG_State): implement the framebuffer parameter getters and setters

glFramebufferParameteri, glGetFramebufferParameteriv and their two by-name
siblings were all export stubs - the GL_ARB_framebuffer_no_attachments entry
points. The stub raises no error and writes nothing, so
direct_state_access.framebuffers_get_parameter_errors saw GL_NO_ERROR for all
three conditions it checks.

FramebufferObject gains the five DEFAULT_* parameters as real state, initialised
to GL 4.6 core table 23.24 and bumping the object version on a write like the
read buffer does. The getter answers those plus the six derived names -
GL_SAMPLES and GL_SAMPLE_BUFFERS from the attachments' sample counts,
GL_IMPLEMENTATION_COLOR_READ_FORMAT/_TYPE from the read buffer's internal format,
GL_DOUBLEBUFFER true only for the window-system framebuffer, GL_STEREO false
because stereo surfaces are not exposed - which is what glGetIntegerv already
reports for the bound framebuffer.

The pname rules live in ValidateFramebufferParameterPname, and their ORDER is
load-bearing: a name outside the table is INVALID_ENUM, and only a name that IS
in the table but that the default framebuffer cannot answer is INVALID_OPERATION.
Testing the framebuffer kind first would answer INVALID_ENUM for
GL_FRAMEBUFFER_DEFAULT_WIDTH on framebuffer zero, which is exactly the third
thing the case checks. The by-name forms take zero as the default framebuffer,
like the other DSA framebuffer entry points.

Rendering to a framebuffer with no attachments is deliberately NOT enabled by
this: CheckCompleteness still reports INCOMPLETE_MISSING_ATTACHMENT, because no
backend can rasterize one. The state is real and the queries are honest; the
draw path is a separate piece of work.

Takes framebuffers_get_parameter_errors from failing to passing on both backends,
with framebuffers_get_parameters - which passed only because both getters were
stubs leaving the CTS's zero-initialised comparands untouched - still passing.

MobileGL/MG_State/GLState/FramebufferState/FramebufferObject.cpp

@@ -185,6 +185,20 @@ namespace MobileGL::MG_State::GLState {
         return m_externalIndex;
     }
 
+#define MOBILEGL_DEFINE_FRAMEBUFFER_DEFAULT_SETTER(name, member, type)                                                \
+    void FramebufferObject::Set##name(type value) {                                                                    \
+        if (member == value) return;                                                                                    \
+        member = value;                                                                                                 \
+        ++m_objectVersion;                                                                                              \
+    }
+
+    MOBILEGL_DEFINE_FRAMEBUFFER_DEFAULT_SETTER(DefaultWidth, m_defaultWidth, Int)
+    MOBILEGL_DEFINE_FRAMEBUFFER_DEFAULT_SETTER(DefaultHeight, m_defaultHeight, Int)
+    MOBILEGL_DEFINE_FRAMEBUFFER_DEFAULT_SETTER(DefaultLayers, m_defaultLayers, Int)
+    MOBILEGL_DEFINE_FRAMEBUFFER_DEFAULT_SETTER(DefaultSamples, m_defaultSamples, Int)
+    MOBILEGL_DEFINE_FRAMEBUFFER_DEFAULT_SETTER(DefaultFixedSampleLocations, m_defaultFixedSampleLocations, Bool)
+#undef MOBILEGL_DEFINE_FRAMEBUFFER_DEFAULT_SETTER
+
     void FramebufferObject::BumpAttachmentVersion(FramebufferAttachmentType type) {
         ++m_attachmentVersions[static_cast<SizeT>(type)];
         ++m_objectVersion;

MobileGL/MG_State/GLState/FramebufferState/FramebufferObject.h

@@ -129,6 +129,19 @@ namespace MobileGL {
             void SetReadBuffer(FramebufferAttachmentType buf);
             FramebufferAttachmentType GetReadBuffer() const { return m_readBuffer; }
 
+            // GL_ARB_framebuffer_no_attachments state (GL 4.6 core table 23.24). The shape a
+            // framebuffer with no attachments would rasterize at; all zero / FALSE until set.
+            Int GetDefaultWidth() const { return m_defaultWidth; }
+            Int GetDefaultHeight() const { return m_defaultHeight; }
+            Int GetDefaultLayers() const { return m_defaultLayers; }
+            Int GetDefaultSamples() const { return m_defaultSamples; }
+            Bool GetDefaultFixedSampleLocations() const { return m_defaultFixedSampleLocations; }
+            void SetDefaultWidth(Int value);
+            void SetDefaultHeight(Int value);
+            void SetDefaultLayers(Int value);
+            void SetDefaultSamples(Int value);
+            void SetDefaultFixedSampleLocations(Bool value);
+
             FramebufferAttachmentVersionArray GetAllFramebufferAttachmentVersions() const {
                 return m_attachmentVersions;
             }
@@ -148,6 +161,12 @@ namespace MobileGL {
             FramebufferAttachmentArray m_drawBuffers; // Probably no versioning needed for this, just check equality
             FramebufferAttachmentType m_readBuffer = FramebufferAttachmentType::None;
 
+            Int m_defaultWidth = 0;
+            Int m_defaultHeight = 0;
+            Int m_defaultLayers = 0;
+            Int m_defaultSamples = 0;
+            Bool m_defaultFixedSampleLocations = false;
+
             // This version will bump when draw/read buffer changes (by `glDrawBuffer(s)`/`glReadBuffer`)
             Uint16 m_objectVersion = 0;
         };