Loosen requirements for mixed samples support

Quits requiring EXT_raster_multisample and
NV_sample_mask_override_coverage for mixed samples support. This will
allow platforms without those latter extensions (i.e. Chrome) to still
use mixed samples for path rendering. Also moves the mixed samples cap
out of shader caps, since it no longer denotes shader functionality.

BUG=skia:

Review URL: https://codereview.chromium.org/1410383011
4 files changed