vga: optimize horizontal pel panning in 256-color modes

Do not go through the panning buffer unless the address wraps in the middle
of the line.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

hw/display/vga-helpers.h

@@ -265,6 +265,18 @@ static void *vga_draw_line8d2(VGACommonState *vga, uint8_t *d,
 
     palette = vga->last_palette;
     hpel = (hpel >> 1) & 3;
+
+    /* For 256 color modes, we can adjust the source address and write directly
+     * to the destination, even if horizontal pel panning is active.  However,
+     * the loop below assumes that the address does not wrap in the middle of a
+     * plane.  If that happens...
+     */
+    if (addr + (width >> 3) * 4 < VGA_VRAM_SIZE) {
+        addr += hpel * 4;
+        hpel = 0;
+    }
+
+    /* ... use the panning buffer as in planar modes.  */
     if (hpel) {
         width += 8;
         d = vga->panning_buf;