Avoid directly manipulating XImage data for now.

The previous tests to detect RGB8 appear to have been insufficient.
This may have resulted in various artifacts such as transparency with
some drivers on some hardware in X11 mode.

src/conio/x_events.c

@@ -71,6 +71,7 @@ static Display *dpy=NULL;
 static Window win;
 static Cursor curs = None;
 static Visual visual;
+static bool VisualIsRGB8 = false;
 static XImage *xim;
 static XIM im;
 static XIC ic;
@@ -284,6 +285,13 @@ static int init_window()
 	}
 	if (best != -1) {
 		visual = *vi[best].visual;
+		/*
+		 * TODO: Set VisualIsRGB8 if appropriate...
+		 * "appropriate" in this context means it's a sequence of
+		 * unpadded uint32_t values in XXRRGGBB format where XX is
+		 * ignored, and RR, GG, and BB are Red, Green, Blue values
+		 * respectively.
+		 */
 		base_pixel = ULONG_MAX;
 		base_pixel &= ~visual.red_mask;
 		base_pixel &= ~visual.green_mask;
@@ -477,7 +485,6 @@ local_draw_rect(struct rectlist *rect)
 	int idx;
 	uint32_t last_pixel = 0x55555555;
 	struct graphics_buffer *source;
-	bool isRGB8 = false;
 
 	if (bitmap_width != rect->rect.width || bitmap_height != rect->rect.height) {
 		bitmap_drv_free_rect(rect);
@@ -513,8 +520,6 @@ local_draw_rect(struct rectlist *rect)
 
 	/* TODO: Translate into local colour depth */
 	idx = 0;
-	if (visual.red_mask == 0xff0000 && visual.green_mask == 0x00ff00 && visual.blue_mask == 0x0000ff)
-		isRGB8 = true;
 
 	for (y = 0; y < source->h; y++) {
 		for (x = 0; x < source->w; x++) {
@@ -534,7 +539,7 @@ local_draw_rect(struct rectlist *rect)
 					continue;
 				}
 			}
-			if (isRGB8) {
+			if (VisualIsRGB8) {
 				pixel = source->data[idx];
 				((uint32_t*)xim->data)[idx] = pixel;
 			}