Cache accesses to VRAM (I_Flip 3.4x)

Really makes you wonder how many hours are really needed to get things
to line up properly

.gitignore

@@ -2,3 +2,4 @@ build-cg/
 build-sdl2/
 wad/
 wad-edited/
+notes/

cgdoom/i_video.c

@@ -17,7 +17,7 @@
 // $Log:$
 //
 // DESCRIPTION:
-//	DOOM graphics stuff for Nspire
+//	DOOM graphics stuff for fx-CG 50
 //
 //-----------------------------------------------------------------------------
 

src-cg/platform.h

@@ -74,7 +74,9 @@ extern int CGD_TrustUnalignedLumps;
    use the secondary VRAM as heap. We swap because using the VRAM as heap would
    cause problems with error screens (stepping through errors would crash). */
 #ifdef CGDOOM_DIRECT_R61524
-# define CGDOOM_SCREENS_BASE ((void *)GetVRAMAddress())
+# define CGDOOM_SCREENS_BASE ((void *)( \
+       ((uint32_t)GetVRAMAddress() & 0x1fffffff) | 0x80000000 \
+   ))
 #else
 # define CGDOOM_SCREENS_BASE SaveVRAMBuffer
 #endif