Previously the enum conversion script would convert all enums in gl.xml
including those for desktop GL. When replaying from a CPP file we would
sometimes try to use enum values that only are exposed in desktop GL.

One example is GL_TEXTURE_MAX_ANISOTROPY/GL_TEXTURE_MAX_ANISOTROPY_EXT.
GLES only has the "EXT" version. Desktop uses the core version. This
was breaking a replay sample that used the EXT enum.

Fix this by a) including the anisotropy extension in registry_xml and
b) updating the enum script to only parse GLES 2 entries. In the future
we could improve the situation by auto-generating a header that defines
exactly the enums we need for ANGLE.

Also updates the enum helper to use uppercase enum values.

Bug: angleproject:3611
Change-Id: I1e01608bc8af282dc078d5a19ac4eba659c96b57
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1894143
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Tim Van Patten <timvp@google.com>