Partial cubemap support for WebGPU

[mvaligursky]

• cubemap can be used by the shader, and it can be used as a render target
• upload of texture data into a cubemap is still not supported
• the render target cubemap is flipped upside-down. Setting RenderTarget.flipY to false instead of true (as required by WebGl) fixes this. This will be addressed at a later stage.
• refactor of shader precision code generation set it up for WebGPU shaders
• improved handling of failed-to-compile webgpu shaders by skipping the drawCall, instead of trying to render it and bringing down the whole rendering.

render.to.cubemap.mov