Refactor EGL/GL API loading

Remove glapi.sh code generation, replace it with hand-written loading
code that checks extension strings before calling eglGetProcAddress.

The GLES2 renderer still uses global state because of:

- {PUSH,POP}_GLES2_DEBUG macros
- wlr_gles2_texture_from_* taking a wlr_egl instead of the renderer

backend/rdp/backend.c

@@ -6,7 +6,6 @@
 #include <wlr/render/gles2.h>
 #include <wlr/util/log.h>
 #include "backend/rdp.h"
-#include "glapi.h"
 #include "util/signal.h"
 
 struct wlr_rdp_backend *rdp_backend_from_backend(