Allow the user to set the background clear color during emulation

The background color can be seen at the sides of the bottom screen or when the window is wider than normal.

src/citra/default_ini.h

@@ -41,6 +41,13 @@ gpu_refresh_rate =
 # 0 (default): No frameskip, 1: x2 frameskip, 2: x4 frameskip, 3: x8 frameskip, etc.
 frame_skip =
 
+[Renderer]
+# The clear color for the renderer. What shows up on the sides of the bottom screen.
+# Must be in range of 0.0-1.0. Defaults to 1.0 for all.
+bg_red =
+bg_blue =
+bg_green =
+
 [Data Storage]
 # Whether to create a virtual SD card.
 # 1 (default): Yes, 0: No