Weirdly, the gl3ext.h has no entries currently, and to access
extensions from GLES3 you need to include this GLES2 header.

We now have code-generated piglit-dispatch support for all of desktop,
GLES2, and GLES3.

v2: Express the corename substitution as a regexp. (suggestion by Paul)

Reviewed-by: Paul Berry <stereotype441@gmail.com>