Add IMG_pvric_end_to_end_signature extension for GLSC 2.0

[dpkcareergreatness]

This extension for OpenGL SC 2.0 provides a safety signature mechanism for Imagination Technologies PowerVR Image Compression (PVRIC), that can be used to verify the integrity of the compression and decompression done by PVRIC as well as the corresponding memory accesses data paths, when processing safety-related workloads, in the presence of potential faults.

[pdaniell-nv]

@oddhack, feel free to merge this vendor extension if you are happy with it.

[oddhack]

LGTM.