Refactor color attachment and depth stencil setup

- The new `ReadTextureFromGPU` method is used to copy a `wgpu::Texture` into a `wgpu::Buffer`, then map the buffer to the CPU. The callback function will be invoked with the mapped data.
- The GetPixelData(), GetRGBAPixelData() and GetRGBACharPixel() methods have all been refactored to now leverage `ReadTextureFromGPU`.