EGL: Combine the GL and GLES current contexts (v2)

EGL: Combine the GL and GLES current contexts (v2)

Only keep track of a single current context, instead of separate
contexts for GL and GLES.

In EGL 1.4 (and 1.5), EGL_OPENGL_API and EGL_OPENGL_ES_API are supposed
to be interchangeable for all purposes except for eglCreateContext.

The _EGLThreadInfo::CurrentContexts array is now a single pointer to the
current context, which may be a GL or GLES context. In addition, it now
keeps track of the current API as an enum instead of an index.

eglMakeCurrent will now replace the current context, regardless of which
client API is used for for the current and new contexts. It no longer
checks for a conflicting context. In addition, calling eglMakeCurrent
with EGL_NO_CONTEXT will now release the current context regardless of
the current API.

v2: Rebased against master (Adam Jackson)

Reviewed-by: Adam Jackson <ajax@redhat.com>