- This implements cpu-only mipmaps generation.

To do in the next CLs:
- Cube textures support
- GPU faster path with vkCmdBlitImage
- Find the bug with clearing of level 0 mips.
- Investigate the intel linux specific bug.

Bug: angleproject:2502
Change-Id: Iecd73cb45ebc9c79ce6291c4529eb27330b51e2c
Reviewed-on: https://chromium-review.googlesource.com/1064194
Commit-Queue: Luc Ferron <lucferron@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>