rework a bit how we manage EGL extensions

- don't advertise extensions that are not supported
  by any implementation

- remove EGL_ANDROID_swap_rectangle which is not
  implemented by anybody and confuses people

- add some comments about mandatory extensions

Bug: 5428001
Change-Id: Id8dc48116ac1d1eb79ec9ef55d03e29d4257c1f3
diff --git a/opengl/libagl/egl.cpp b/opengl/libagl/egl.cpp
index 03db8d7..6d4098c 100644
--- a/opengl/libagl/egl.cpp
+++ b/opengl/libagl/egl.cpp
@@ -49,6 +49,11 @@
 #undef NELEM
 #define NELEM(x) (sizeof(x)/sizeof(*(x)))
 
+
+EGLBoolean EGLAPI eglSetSwapRectangleANDROID(EGLDisplay dpy, EGLSurface draw,
+        EGLint left, EGLint top, EGLint width, EGLint height);
+
+
 // ----------------------------------------------------------------------------
 namespace android {
 // ----------------------------------------------------------------------------