backend: gl: don't force fbconfig info on to heap

It's fairly small, so it's reasonable to put it on the stack.

Signed-off-by: Yuxuan Shui <yshuiv7@gmail.com>

src/common.h

@@ -204,7 +204,7 @@ typedef struct session {
 	/// Custom GLX program used for painting window.
 	// XXX should be in struct glx_session
 	glx_prog_main_t glx_prog_win;
-	struct glx_fbconfig_info *argb_fbconfig;
+	struct glx_fbconfig_info argb_fbconfig;
 #endif
 	/// Sync fence to sync draw operations
 	xcb_sync_fence_t sync_fence;