AGS: Make VRAM 8 bits per pixel

Before this, the virtual VRAM was 32 bits per pixel, with each pixel
holding an 8-bit index color or 0x80RRGGBB. The latter was for Ayumi-
chan Monogatari Jisshaban, which has been unsupported since system3-sdl2
was forked from System3 for Win32.

This change removes the 0x80RRGGBB format and makes the virtual VRAM 8
bits per pixel.

src/sys/ags_pms.cpp

@@ -150,7 +150,7 @@ void AGS::load_pms(uint8* data, int page, int transparent)
 
 		// VRAMに転送
 		if(extract_cg) {
-			uint32* dest = &vram[dest_screen][y + sy][sx];
+			uint8_t* dest = &vram[dest_screen][y + sy][sx];
 			if(transparent == -1) {
 				for(int x = 0; x < width; x++) {
 					cgdata[2][x] = cgdata[1][x];