Keep screen textures on GPU during screen capture

Today, bitmaps captured by screen capture are always copied to the CPU before they can be stored to file or presented in a scene. With a 4K monitor (3840x2160), the GPU/CPU transfer rate becomes approx. 2GB/sec at 60 Hz.

These kinds of transfer rates will impact system performance, and can potentially cause problems in performance sensitive use cases such as gaming.

The goal of this task is to allow keeping grabbed textures in GPU memory as long as possible, to facilitate GPU based resizing, compression, to avoid transferring entire screen bitmaps to the CPU memory. 

Example from laptop Intel GPU:

On a discrete nVidia GPU, the problem is not as visible.