We had a performance regression in the Textures benchmark. What the
test was doing was iterating over all possible texture state,
ensuring the active texture was dirty every frame. This is an attempt
to improve on the speed by not doing as much resetting work in
State::syncProgramTextures. It introduces an active textures mask to
speed iteration over the active texture set.

Also makes a refactoring change to Context to make it easier to limit
caps to an implementation maxium. The number of active textures is
limited to 64 so they easily fit in the bitset mask, with a limit of
32 per shader stage. No mask is currenly kept for compute shaders.

With the fix the performance should be about the same as before (which
is good, as the test always sets the textures dirty).

Test: TexturesBenchmark.Run/gl_8_textures_5_rebind_3_state_8_mips

BUG=chromium:765363
BUG=angleproject:1387

Change-Id: I8bcf95be3671195373573f89f406edaba40aa1be
Reviewed-on: https://chromium-review.googlesource.com/670279Reviewed-by: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Frank Henigman <fjhenigman@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>