[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/Classifiers/TextureEnumClassifier.cpp

@@ -28,6 +28,7 @@ namespace MobileGL {
 
         bool IsStencilFormatInternalFormat(TextureInternalFormat internalformat) {
             switch (internalformat) {
+            case TextureInternalFormat::StencilIndex8:
             case TextureInternalFormat::Depth24Stencil8:
             case TextureInternalFormat::Depth32FStencil8:
             case TextureInternalFormat::DepthStencil:

MobileGL/MG_Util/Converters/GLToMG/TextureEnumConverter.cpp

@@ -251,6 +251,8 @@ namespace MobileGL {
                 return TextureInternalFormat::Depth24Stencil8;
             case GL_DEPTH32F_STENCIL8:
                 return TextureInternalFormat::Depth32FStencil8;
+            case GL_STENCIL_INDEX8:
+                return TextureInternalFormat::StencilIndex8;
             case GL_DEPTH_COMPONENT:
                 return TextureInternalFormat::DepthComponent;
             case GL_DEPTH_STENCIL:

MobileGL/MG_Util/Converters/MGToGL/TextureEnumConverter.cpp

@@ -233,6 +233,8 @@ namespace MobileGL {
                 return GL_DEPTH24_STENCIL8;
             case TextureInternalFormat::Depth32FStencil8:
                 return GL_DEPTH32F_STENCIL8;
+            case TextureInternalFormat::StencilIndex8:
+                return GL_STENCIL_INDEX8;
             case TextureInternalFormat::DepthComponent32:
                 return GL_DEPTH_COMPONENT32;
             case TextureInternalFormat::DepthStencil:

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:

MobileGL/MG_Util/Converters/MGToVk/TextureEnumConverter.cpp

@@ -226,6 +226,8 @@ namespace MobileGL {
                 return VK_FORMAT_D24_UNORM_S8_UINT;
             case TextureInternalFormat::Depth32FStencil8:
                 return VK_FORMAT_D32_SFLOAT_S8_UINT;
+            case TextureInternalFormat::StencilIndex8:
+                return VK_FORMAT_S8_UINT;
             case TextureInternalFormat::DepthComponent32:
                 return VK_FORMAT_D32_SFLOAT;
             case TextureInternalFormat::DepthStencil:

MobileGL/MG_Util/Metrics/TextureMetrics.cpp

@@ -18,6 +18,7 @@ namespace MobileGL {
             case TextureInternalFormat::Red: // UNorm8 shadow layout
             case TextureInternalFormat::R8Snorm:
             case TextureInternalFormat::R8I:
+            case TextureInternalFormat::StencilIndex8:
             case TextureInternalFormat::R8UI:
                 return 1;
 
@@ -508,6 +509,9 @@ namespace MobileGL {
                 s.Depth = 32;
                 s.Stencil = 8;
                 break;
+            case TextureInternalFormat::StencilIndex8:
+                s.Stencil = 8;
+                break;
             case TextureInternalFormat::Unknown:
                 // Queried for attachments that have no storage yet (e.g. framebuffer
                 // parameter queries on the initial state); every size stays 0.