Add a 'flush' command to graphics subsystem

...I'm actually concerned that I went a bit overkill trying to prevent
backwards compatibility issues with this abstraction design, because
this is a large number of files that have to be modified just to add a
single graphics subsystem export.  Someone's going to strangle me, and
when you know that someone might strangle you, that means that you did
something wrong.  We'll have to look in to simplifying this in the
future without killing backward compatibility safety.

libobs-opengl/gl-subsystem.c

@@ -972,6 +972,11 @@ void device_clear(device_t device, uint32_t clear_flags,
 	UNUSED_PARAMETER(device);
 }
 
+void device_flush(device_t device)
+{
+	glFlush();
+}
+
 void device_setcullmode(device_t device, enum gs_cull_mode mode)
 {
 	if (device->cur_cull_mode == mode)