MGA: Implement gamma-correction for <= 16 bpp modes

src/include/86box/vid_svga.h

@@ -271,6 +271,9 @@ typedef struct svga_t {
     /* Enable LUT mapping of >= 24 bpp modes. */
     int lut_map;
 
+    /* Return a 32 bpp color from a 15/16 bpp color. */
+    uint32_t (*conv_16to32)(struct svga_t *svga, uint16_t color, uint8_t bpp);
+
     void *  dev8514;
     void *  xga;
 } svga_t;