Support for run-time selection between GLES and Desktop GL implementations.

This CL removes the compile-time selection of the GL implementation based on the GR_SUPPORT_GLDESKTOP family of macros.  

Support for the platform is configured dynamically, by querying the fBindingsExported member of active GrGLInterface instance.

Committed:  http://code.google.com/p/skia/source/detail?r=960

[twiz1, 2011-03-18 06:33:18]

PTAL.

Note that this change currently pulls in the changes required to remove
dependency on GL headers.  See here: http://codereview.appspot.com/4272061

The plan is to submit that change, followed by this one.

One area where this change may need improvement is the naming of the
GL_SUPPORT_XXXX macros.  The static ones are GR_SUPPORT_XXX, and the ones that
resolve to the dynamic check are GR_GL_SUPPORT_XXX.  To prevent accidental use I
#undef the static ones outside of the GL configuration code.

Jeff

[bsalomon, 2011-03-18 13:46:30]

Jeff, you're a hero! :)

This looks great. 

I just remembered something we'll want to address in the long term. This isn't
worth holding up Chrome work for, though. On some ES platforms (ios) the headers
and GL runtime are structured so that an app can include both headers. Functions
that are in both ES1 and ES2 are handled at runtime based on the version of the
current context. So I think we really just want ES vs Desktop as opposed to a
tri-state for GrGLBinding. But like I said it isn't important in the short-run.