Use an rwlock to protect against concurrent calls to eglTerminate,
instead of using a secondary refcount.

Removed EplDisplay::use_count, and instead added EplDisplay::init_lock,
which is a read/write lock that protects the eglInitialize count.

eglInitialize, eglTermiante, and teardown will all take the write lock.

Any function which needs an initialized EGLDisplay will take the read
lock. That's enough to prevent any other threads from terminating the
EGLDisplay out from under it, without otherwise blocking concurrent
functions.

As with surfaces, a read lock is fine. The surface list has its own
lock, and outside of eglInitialize/eglTerminate, everything else in
EplDisplay has its own lock.

In particular, eglSwapBuffers implementations no longer need to
temporarily unlock the display before perfoming long waits or reads.

Unlocking like that makes it much harder to reason about, because you
have to deal with the case where another thread calls eglDestroySurface
or eglTerminate.

In addition, the use_count field would give the wrong behavior in one
(admittedly contrived) case: If another thread calls eglTerminate and
then eglInitilize during that temporary unlock, then they get
effectively ignored, and none of the cleanup stuff in eglTerminate
happens.

Add an explicit dependency on 'egl' to the meson.build files.

Remove eplGetCurrentDisplay, and replace it with a eplGetCurrentSurface
function that returns both the current EGLDisplay and the current
EGLSurface.

That makes it possible to use eplHookDisplaySurface and
eplHookDisplaySurfaceEnd with functions that operate on the current
EGLSurface.

Add an optional EplImplFuncs::SwapInterval function to handle
eglSwapInterval calls.

If it's not NULL, then the base library will provide a hook function for
eglSwapInterval.

HookSwapInterval will handle looking up the current EGLDisplay and
EGLSurface and making sure that they're valid, including all the
necessary locking.

Removed all of the temporary unlocking in eglSwapBuffers. Now that we
use an rwlock, that's no longer necessary.

Updated eplX11CreateWindowSurface and eplX11CreatePixmapSurface to take
the existing surface list as a parameter.

Merged eplX11FreeWindow into eplX11DestroyWindow.

Changed to use EplImplFuncs::SwapInterval instead of providing the
entire hook function.