[Fix] (DirectGLES): report distinct depth/stencil framebuffers as unsupported

GL only requires framebuffers whose depth and stencil attachments refer to the
same image; anything else may be answered GL_FRAMEBUFFER_UNSUPPORTED, and both
backends' real targets do exactly that - DirectVulkan cannot form two separate
attachments at all, and the ES drivers behind DirectGLES return UNSUPPORTED for
a separate depth renderbuffer plus stencil renderbuffer.

The frontend already knew how to detect the configuration, but only consulted it
for DirectVulkan. On DirectGLES it answered GL_FRAMEBUFFER_COMPLETE for a
framebuffer the driver had rejected, so every clear and draw against it was
silently dropped and the results read back as zeros - which is what
KHR-GL3x.packed_depth_stencil.verify_mixed_attachments saw. (That test
explicitly tolerates GL_FRAMEBUFFER_UNSUPPORTED; what it cannot survive is being
told the framebuffer works.)

Turned into a backend capability rather than a backend-type check, probed once
at init from a scratch framebuffer the same way the format-capability cache is,
so a driver that does support the configuration keeps using it. Defaults to
supported, leaving any backend that does not set it on the permissive path.

Fixes KHR-GL3{2,3}.packed_depth_stencil.verify_mixed_attachments for both
formats; DirectVulkan re-verified unchanged at 23/25 pass + 2 not-supported.

MobileGL/MG_Backend/BackendObject.h

@@ -324,6 +324,13 @@ namespace MobileGL {
             Float MaxFragmentInterpolationOffset = 0.4375f;
             Int FragmentInterpolationOffsetBits = 4;
             Bool SupportsWideLines = false;
+            // Whether a framebuffer whose depth and stencil attachments are distinct
+            // images can be rendered to. GL only requires support when both refer to the
+            // same image and lets an implementation answer GL_FRAMEBUFFER_UNSUPPORTED
+            // otherwise, which is what DirectVulkan (one combined attachment) and the
+            // real ES drivers behind DirectGLES both do. Defaults to true so a backend
+            // that never sets it keeps the permissive behaviour.
+            Bool SupportsDistinctDepthStencilAttachments = true;
             SizeT MaxShaderStorageBlockSize = 128 * 1024 * 1024;
             Uint32 SubgroupSize = 0;
             Uint32 SubgroupSupportedStages = 0;