Add a new context.c file with context_enter and context_leave functions

So far, this just factors out some duplicated code from glxwrap.c and
eglwrap.c into the new context_enter/leave functions.

Eventually, some of the code currently living in metrics.c should
migrate up into context.c, (such as the global current_context
variable in metrics.c).

Additionally, the context.c layer will give us a natural place to
query things such as "is the AMD_performance_monitor extension
available?".