- Added *ALPHA8*/*LUMINANCE8* formats and *_SNORM formats
  to non color renderable formats
- Added a new function IsMipMappable in order to allow
  mipmapping of the *ALPHA8*/*LUMINANCE8* formats

Fixes all (24) failures in:
dEQP-GLES3.functional.texture.mipmap.2d.generate*
dEQP-GLES3.functional.texture.mipmap.cube.generate*

Also fixes WebGL test:
conformance/textures/misc/texture-npot.html

Change-Id: I5f3210094fbc5b2e5bae25c88a5ef4f1ffb69cbd
Reviewed-on: https://swiftshader-review.googlesource.com/14130Tested-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>