Preload command speedup, Texture/buffer data flush, blit shader fix

[riperiperi]

• Tie current encoder to command buffer. This allows extra command buffers to mainain their own encoder state without interrupting the main encoder.

  • This fixes an issue where buffer data preload would randomly split the render pass in the main thread.

• Support background context actions, specifically Texture GetData and SetData methods.

  • Implemented CopyFromOrToBuffer for textures. Should probably use it for other methods to avoid duplication, but it could be error prone so I put it off for now.

• Fix an issue where helper shaders that used render targets weren't ending their render pass when returning to the main state. Some helpers (clear color/depth) can skip doing this.

• Doesn't currently support scaled copy from background thread (it's skipped). Required for resolution scale.