Reject 2, 3, 5, 6, 7-bpp BMP images

BMP decoder assumes less than 8 bit depth images have 1 or 4 bits
per pixel. No other depths are correctly translated to an 8bpp
surface.

This patch rejects loading these images.

https://bugzilla.libsdl.org/show_bug.cgi?id=4498
Signed-off-by: Petr P?sa? <ppisar@redhat.com>

src/video/SDL_bmp.c

@@ -222,6 +222,14 @@ SDL_Surface * SDL_LoadBMP_RW (SDL_RWops *src, int freesrc)
 					break;
 			}
 			break;
+		case 2:
+		case 3:
+		case 5:
+		case 6:
+		case 7:
+			SDL_SetError("%d-bpp BMP images are not supported", biBitCount);
+			was_error = SDL_TRUE;
+			goto done;
 		default:
 			SDL_SetError("Compressed BMP files not supported");
 			was_error = SDL_TRUE;