[Feat] (MG_Impl, MG_Util): STENCIL_INDEX8 renderbuffers; report distinct D/S renderbuffers unsupported

GL_STENCIL_INDEX8 becomes a first-class internal format (VK_FORMAT_S8_UINT
backing, metrics, classifiers, converters), so glRenderbufferStorage
accepts it instead of leaving GL_INVALID_ENUM behind. Framebuffer
completeness now also mirrors the renderer's gate for renderbuffers:
distinct depth/stencil renderbuffer attachments (or a renderbuffer
paired with a texture) report GL_FRAMEBUFFER_UNSUPPORTED - the spec only
requires the same-image case - instead of passing completeness and then
failing at draw/clear (verify_mixed_attachments.* now passes).

MobileGL/MG_Util/Converters/MGToStr/TextureEnumConverter.cpp

@@ -234,6 +234,8 @@ namespace MobileGL {
                 return "Depth24Stencil8";
             case TextureInternalFormat::Depth32FStencil8:
                 return "Depth32FStencil8";
+            case TextureInternalFormat::StencilIndex8:
+                return "StencilIndex8";
             case TextureInternalFormat::Red:
                 return "Red";
             case TextureInternalFormat::RG: