sokol_app.h gl: make GL context current in NSOpenGLView.reshape

Not sure if this is a regression in macOS, or never worked:

The SAPP_EVENTTYPE_RESIZED event may be fired from within the
NSOpenGLView.reshape method, which doesn't seem to have a GL
context bound, so calling any GL function from within
the RESIZED handler fails. The commit simply makes the
NSOpenGLView's GL context current in the reshape function.

sokol_app.h

@@ -3983,6 +3983,7 @@ _SOKOL_PRIVATE void _sapp_macos_frame(void) {
     the correct dimensions.
 */
 - (void)reshape {
+    [[self openGLContext] makeCurrentContext];
     _sapp_macos_update_dimensions();
     [super reshape];
 }