gldownloadelement: accelerate with glExportTexImageDMA

Accelerate GL texture download with glExportTexImageDMA if driver
supports it.

On columbia, dowloading 720p textures:
* RGBA performace up from 28 fps to 108 fps (3.8x improvement)
* RGB performance up from 2 fps to 48 fps (24x improvement)

Enterprise sees performance improvments too but they're less drastic
as it's a much powerful platform overall.
Measurements based on otherwise idle system, add more workloads such
as on screen rendering and performance increase will not be as

BUG: 146907014
Change-Id: Idced5ea3aeae835187461bef0cf0d9113f27bfe3
2 files changed