trectangle: fix draw rects sometimes being inaccurate (fixup a30a627)

Apparently, the manual rect calculations sometimes (eg, with the Progress Bar widget)
resulted in incorrectly placed rectangles being drawn. Using the bare dimension
values fixes this problem.

src/gui/auxiliary/canvas.cpp

@@ -808,12 +808,7 @@ void trectangle::draw(surface& canvas,
 
 	if(fill_color_) {
 
-		const unsigned left = x + border_thickness_;
-		const unsigned right = left + w - (2 * border_thickness_) - 1;
-		const unsigned top = y + border_thickness_;
-		const unsigned bottom = top + h - (2 * border_thickness_);
-
-		sdl::draw_solid_tinted_rectangle(left, top, right, bottom,
+		sdl::draw_solid_tinted_rectangle(x, y, w, h,
 			(fill_color_ & 0xFF000000) >> 24,
 			(fill_color_ & 0x00FF0000) >> 16,
 			(fill_color_ & 0x0000FF00) >> 8,