BUG=angleproject:2306

This implements GL_OES_draw_texture using a few bits of new state
in the renderer and adding a code path for it in the shader, using
gl_VertexID to draw the quad backing the texture draw. This allows us
to avoid allocating a separate vertex array for the texture draw and
reuses the current shader as much as possible, plugging in to the
existing multitexturing pipeline.

- Add unit test and sample
- No new test expectations, but advertising GL_OES_draw_texture makes
the DrawTex GLES1 conformance test non-trivial and actually test
glDrawTex*.

Change-Id: I1485098249fe44d46a01cab4bb7b2c39d0492923
Reviewed-on: https://chromium-review.googlesource.com/1135930Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Lingfeng Yang <lfy@google.com>