Fix UV issues in Compatibility renderer

[moonpirates]

Fixes a bug where CanvasTexture's UVs were off when using the Compatibility mode's renderer.

Fixes #86746

[AThousandShips]

I'd suggest against breaking up the if/else block, it makes sense as it were, just fixing the size in the default case, like so:

if (!texture) {
	ct->diffuse = texture_storage->texture_gl_get_default(GLES3::DEFAULT_GL_TEXTURE_WHITE);
	size_cache = Size2i(1, 1);
} else {
	size_cache = Size2i(texture->width, texture->height);
}

And remove the rest

[moonpirates]

@AThousandShips the problem with that is that the diffuse slot, upon a subsequent iteration, is assigned to a texture (the default white), making it end up in the else statement, which will fall back to the 4x4 size.

Thanks for your feedback, I'll fix that!

[clayjohn]

Looks great!

[akien-mga]

Thanks! And congrats for your first merged Godot contribution 🎉