This refactoring is done to prepare for adding more mipmap tests for
ES3 where base level / max level is changed from 0.

1. Reuse drawQuad() from ANGLETest instead of having completely custom
quad drawing code, and reuse the same clear+draw function for both 2D
and cube map texture tests.

2. Add a helper function for clearing texture level 0 with glClear.

3. Remove a few redundant texture binding and useProgram calls.

4. Rely on sampler uniforms being set to 0 by default and texture unit
0 being active by default.

5. Remove "Offscreen" from variable names.

6. Split SetUp functions to helper functions.

7. Reuse FillWithRGBA from TextureTest also in MipmapTest.

BUG=angleproject:596
TEST=angle_end2end_tests

Change-Id: I9fd738f4b8b0a19c7aa1d267f7dbaa40a6935631
Reviewed-on: https://chromium-review.googlesource.com/338791Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>