gldownload: accelerate with dma-buf textures

For the formats supported by the GPU stack, allocate dma-bufs for
the texture to download and have upstream render to it. Downloading
then becomes the task to sync dma-buf access before CPU acces (mmap).

Note:
Currently driver supports only RGBA/BGRA, not RGB/BGR.

Change-Id: I04d45d2ba0780a60f5be22b5a48a4c8ba6ca69c7