Change all surface-clearing FillRect()s to use ClearSurface()

ClearSurface() is less verbose than doing it the old way, and also
conveys intent clearer. Plus, some of these FillRect()s had hardcoded
width and height values, whereas ClearSurface() doesn't - meaning this
change also has better future-proofing, in case the widths and heights
of the surfaces involved change in the future.

desktop_version/src/preloader.cpp

@@ -110,7 +110,7 @@ void preloaderrender(void)
   }else if (pre_transition <= -10) {
     //Switch to TITLEMODE (handled by preloaderrenderfixed)
   }else if (pre_transition < 5) {
-    FillRect(graphics.backBuffer, 0, 0, 320,240, graphics.getBGR(0,0,0));
+    ClearSurface(graphics.backBuffer);
   }else if (pre_transition < 20) {
     pre_temprecty = 0;
     pre_temprecth = 240;