clarify resize and stretch logic

cppcheck (via Codacy) rightly noted that the construct ((!!w ^ !!h)) was confusing.

Rearrange the code a bit, note some things must be true without testing them, and we can eliminate two variables, make the code faster than the "cute optimization" and make it FAR easier to read.

src/gui/core/canvas.cpp

@@ -1105,33 +1105,30 @@ void image_shape::draw(surface& canvas,
 	surface surf;
 
 	// Test whether we need to scale and do the scaling if needed.
-	if(w || h) {
-		bool done = false;
-		bool stretch_image = (resize_mode_ == stretch) && (!!w ^ !!h);
-		if(!w) {
-			if(stretch_image) {
+	if ((w == 0) && (h == 0)) {
+		surf = image_;
+	}
+	else { // assert((w != 0) || (h != 0))
+		if(w == 0) { // assert(h != 0)
+			if(resize_mode_ == stretch) {
 				DBG_GUI_D << "Image: vertical stretch from " << image_->w << ','
 						  << image_->h << " to a height of " << h << ".\n";
 
 				surf = stretch_surface_vertical(image_, h);
-				done = true;
 			}
 			w = image_->w;
 		}
-
-		if(!h) {
-			if(stretch_image) {
+		else if(h == 0) { // assert(w != 0)
+			if(resize_mode_ == stretch) {
 				DBG_GUI_D << "Image: horizontal stretch from " << image_->w
 						  << ',' << image_->h << " to a width of " << w
 						  << ".\n";
 
 				surf = stretch_surface_horizontal(image_, w);
-				done = true;
 			}
 			h = image_->h;
 		}
-
-		if(!done) {
+		else { // assert((w != 0) && (h != 0))
 
 			if(resize_mode_ == tile) {
 				DBG_GUI_D << "Image: tiling from " << image_->w << ','
@@ -1157,8 +1154,6 @@ void image_shape::draw(surface& canvas,
 		}
 		src_clip.w = w;
 		src_clip.h = h;
-	} else {
-		surf = image_;
 	}
 
 	if(vertical_mirror_(local_variables)) {