pushGPUCanvas abstract method
Creates a GPUCanvas and pushes it onto the Lua stack.
Returns 1 on success, pushes nil and returns 1 on failure.
The canvas is a 1× presentation target only; MSAA color + depth
are user-allocated via GPUTexture.new and passed in the
context:beginRenderPass(desc) descriptor.
Implementation
int pushGPUCanvas(int width, int height);