[Feat] (MG_State, MG_Impl, MG_Backend, MG_Util): make the border colour real sampler state

glGetSamplerParameterfv(sampler, GL_TEXTURE_BORDER_COLOR) raised INVALID_ENUM,
because MobileGL kept the border colour on the texture object and
GetSamplerParam_State had no case for it at all. That is the first thing
direct_state_access.samplers_defaults asks, so the case threw before reaching
any of the defaults it was written to check.

GL 4.6 core table 23.18 lists TEXTURE_BORDER_COLOR as sampler state, so it moves
to SamplerParameters and TextureObjectBase reaches it through the SamplerObject
it already owns - one source of truth, and a sampler object bound over a texture
now supplies its own border colour, which is what GL says should happen. The
texture params version still moves on a write, because the DirectGLES texture
sync memoises on it. glSamplerParameter{fv,Iiv,Iuiv} and their getters read and
write all four components in whichever representation the caller used, and the
three representations are kept in step so any getter has an answer. The bogus
[0,1] and [0,255] range checks are gone: GL clamps a border colour when a
fixed-point format is sampled, it does not reject it.

DirectVulkan's ResolveVkBorderColor now reads the sampler rather than the
texture. DirectGLES gained a glSamplerParameterfv in its sampler sync, and both
that and the pre-existing glTexParameterfv are gated on a new
SupportsTextureBorderClamp capability - ES 3.2 core, or EXT/OES_texture_border_clamp
before it - since without the extension every such call is INVALID_ENUM on the
driver. DriverPost gains the matching row per the POST rule, saying what a user
actually loses when it is missing.

Takes direct_state_access.samplers_defaults from failing to passing on both
backends.

MobileGL/MG_Util/SelfTest/DriverPost.cpp

@@ -298,6 +298,17 @@ namespace MobileGL::MG_Util::SelfTest {
                              "not supported; no impact: the native indirect path deliberately does not "
                              "rely on it (shader-side emulation handles baseInstance semantics)");
             }
+            if (caps.SupportsTextureBorderClamp) {
+                builder.Pass("Texture border clamp",
+                             "supported (GL_TEXTURE_BORDER_COLOR reaches the driver, so "
+                             "GL_CLAMP_TO_BORDER samples the colour the application set)");
+            } else {
+                builder.Warn("Texture border clamp",
+                             "not supported (pre-ES 3.2 without GL_EXT/OES_texture_border_clamp); "
+                             "GL_TEXTURE_BORDER_COLOR is not synced to the driver at all, so anything "
+                             "sampling outside a GL_CLAMP_TO_BORDER texture reads the driver's default "
+                             "border instead of the requested colour");
+            }
             if (glesFuncs.glPatchParameteri != nullptr) {
                 builder.Pass("Tessellation patch parameters",
                              "glPatchParameteri present (GL_PATCH_VERTICES reaches the driver)");