[Feat] (MG_State, MG_Impl/GLImpl): per-target default texture objects (name 0) - binding 0 binds a real per-context default object (the initial binding of every unit/target slot, rebound on delete of a bound texture), so glTexImage*/glTexParameter*/glGetTex* on it work like any texture while glIsTexture(0)/Gen/Delete keep excluding it and TexStorage* rejects it per spec; backends skip image-less defaults as cheaply as the old null slots (DirectGLES per-draw sync/bind loops, DirectVulkan sampler-fallback resolve); also accept the full advertised GL_MAX_COMBINED_TEXTURE_IMAGE_UNITS range in glActiveTexture, allow zero-layer TexImage3DMultisample, and let glTexBuffer(buffer=0) detach - the texture section of GL CTS per-case state reset (gluStateReset) now runs clean

MobileGL/MG_State/GLState/Core.h

@@ -95,6 +95,8 @@ namespace MobileGL {
                 // Texture
                 void GenTextureNames(Uint number, Vector<Uint>& textures);
                 const SharedPtr<ITextureObject>& GetTextureObject(Uint index);
+                // Per-target default texture object (name 0); see TextureState::GetDefaultTextureObject.
+                const SharedPtr<ITextureObject>& GetDefaultTextureObject(TextureTarget target) const;
                 const SharedPtr<ITextureObject>& CreateTextureObject(Uint index, TextureTarget target);
                 void MarkTextureObjectForDeletion(Uint index);
                 TextureUnit& GetTextureUnitObject(Int unit);