Restore the V_Clear function

I removed it before because I had missed the point about clearing the
screen when changing "details" (= the level of detail).

README

@@ -12,7 +12,6 @@ CGDOOM is licensed under GPLv2 as a derivative work of both the original DOOM
 sources (GPL) and nDoom (GPLv2). See LICENSE.
 
 TODO:
--> Fix screen not cleared when changing resolution
 -> Shareware WAD crashes at the end of E1M4 (and in E1M9)
 -> Ultimate DOOM WAD runs out of memory at the end of E1M2
 -> Supply more VRAM memory to internal allocator

cgdoom/r_main.c

@@ -558,6 +558,8 @@ void R_SetViewSize( int		blocks)
   setblocks = blocks;
 }
 
+void V_Clear();
+
 //
 // R_ExecuteSetViewSize
 //
@@ -571,6 +573,8 @@ void R_ExecuteSetViewSize (void)
   int		startmap; 	
 
   setsizeneeded = false;
+  /* CGDOOM-specific function */
+  V_Clear();
 
   if (setblocks == 11)
   {

cgdoom/v_video.c

@@ -366,3 +366,9 @@ void V_Init (void)
   //Done since loading on real hardware takes so damn long
 //  I_PrepScreen();
 }
+
+/* CGDOOM-specific function to clear screen when changing window size */
+void V_Clear (void)
+{
+  memset(screens[0], 0, 320*(200-32));
+}