[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_Impl/GLImpl/Texture/Validators.cpp

@@ -360,6 +360,19 @@ namespace MobileGL::MG_Impl::GLImpl::TextureImpl {
         return true;
     }
 
+    Bool ValidateTextureNotDefault(const SharedPtr<MG_State::GLState::ITextureObject>& textureObject,
+                                   const char* caller) {
+        if (textureObject && textureObject->GetExternalIndex() == 0) {
+            MG_State::pGLContext->RecordError(
+                ErrorCode::InvalidOperation,
+                MakeUnique<GenericErrorInfo>("MG_Impl/GLImpl", caller,
+                                             "This operation is not allowed on the default texture (zero is "
+                                             "bound to the target)."));
+            return false;
+        }
+        return true;
+    }
+
     Bool ValidateTextureTargetUniformity(SharedPtr<MG_State::GLState::ITextureObject> textureObject,
                                          TextureTarget target) {
         if (!textureObject) return true; // should be created later