Move three extensions to community approved.

Based on consensus on the public-webgl list, the following three
extensions are advanced to community approved:
- ANGLE_instanced_arrays
- OES_texture_float_linear
- OES_texture_half_float_linear
Merge pull request #337 from grorg/community-approval

Move ANGLE_instanced_arrays, OES_texture_float_linear and OES_texture_half_float_linear extensions to community approved.