[GTK] Fix GC#copyArea() for overlapping source and target areas eclipse-platform#1756

The GC provides a #copyArea() method that copies an area inside the
surface of the GC to another area in the same surface. When using a
surface directly created via the Cairo API (or with the GDK API on
Wayland), this operation is broken.
When the source and target area are overlapping and the source is
above-left of the target area, the source area will appear replicated in
the target area, as Cairo does not perform any buffering but does a
linewise/chunkwise write from the source area to the target area.

This change fixes the behavior of GC#copyArea() by explicitly buffering
the area to be copied. It also removes the existing workaround in images
that avoids the instantiation of a surface via the Cairo API, as the
reason for that workaround was the issue fixed by this change.

Fixes eclipse-platform#1756

Author: HeikoKlare

bundles/org.eclipse.swt/Eclipse SWT/gtk/org/eclipse/swt/graphics/GC.java

@@ -545,10 +545,29 @@ void copyAreaInPixels(int srcX, int srcY, int width, int height, int destX, int
 	if (deltaX == 0 && deltaY == 0) return;
 	long drawable = data.drawable;
 	if (data.image != null) {
-		Cairo.cairo_set_source_surface(handle, data.image.surface, deltaX, deltaY);
+		// When filling a part of a surface with another part of a surface, Cairo does not buffer
+		// the whole data to copy but performs some chunk-wise copy from the top left to the bottom right.
+		// Thus if the source and the target area are overlapping, Cairo may copy a chunk into a target
+		// area that is then copied on later, leading to unwanted replications of that source chunk in
+		// the target area.
+		// To avoid this, we buffer the area to copy in a separate surface.
+		int imageFormat = Cairo.cairo_image_surface_get_format(data.image.surface);
+		long tempSurface = Cairo.cairo_image_surface_create(imageFormat, width, height);
+		long tempGC = Cairo.cairo_create(tempSurface);
+		int sourceX = deltaX > 0 ? 0 : deltaX;
+		int sourceY = deltaY > 0 ? 0 : deltaY;
+		Cairo.cairo_set_source_surface(tempGC, data.image.surface, sourceX, sourceY);
+		Cairo.cairo_rectangle(tempGC, 0, 0, width, height);
+		Cairo.cairo_set_operator(tempGC, Cairo.CAIRO_OPERATOR_SOURCE);
+		Cairo.cairo_fill(tempGC);
+		Cairo.cairo_destroy(tempGC);
+		int targetX = deltaX < 0 ? 0 : deltaX;
+		int targetY = deltaY < 0 ? 0 : deltaY;
+		Cairo.cairo_set_source_surface(handle, tempSurface, targetX, targetY);
 		Cairo.cairo_rectangle(handle, destX, destY, width, height);
 		Cairo.cairo_set_operator(handle, Cairo.CAIRO_OPERATOR_SOURCE);
 		Cairo.cairo_fill(handle);
+		Cairo.cairo_surface_destroy(tempSurface);
 	} else if (drawable != 0) {
 		Cairo.cairo_save(handle);
 		Cairo.cairo_rectangle(handle, destX, destY, width, height);

bundles/org.eclipse.swt/Eclipse SWT/gtk/org/eclipse/swt/graphics/Image.java

@@ -1260,11 +1260,7 @@ void init(int width, int height) {
 	this.type = SWT.BITMAP;
 
 	/* Create the pixmap */
-	if (GTK.GTK4) {
-		surface = Cairo.cairo_image_surface_create(Cairo.CAIRO_FORMAT_RGB24, width, height);
-	} else {
-		surface = GDK.gdk_window_create_similar_surface(GDK.gdk_get_default_root_window(), Cairo.CAIRO_CONTENT_COLOR, width, height);
-	}
+	surface = Cairo.cairo_image_surface_create(Cairo.CAIRO_FORMAT_ARGB32, width, height);
 	if (surface == 0) SWT.error(SWT.ERROR_NO_HANDLES);
 	// When we create a blank image we need to set it to 100 in GTK3 as we draw using 100% scale.
 	// Cairo will take care of scaling for us when image needs to be scaled.