GUI2/Canvas: refactor surface blitting out of drawing routines

This entirely refactors surface blitting out of the canvas. Instead, each canvas owns a texture. This texture
is completely redrawn any time the canvas if marked dirty, else the result is cached and drawn to the screen
each draw cycle.

All windows are now redrawn every draw cycle. The use of the cached canvas textures means there's no noticeable
performance difference (likely a performance gain, actually) from using surface blitting.

There's still some code to clean up and a few things to fix.

data/gui/widget/button_default.cfg

@@ -30,8 +30,8 @@
 	[image]
 		x = 2
 		y = 2
-		w = "(width)"
-		h = "(height)"
+		w = "(width - 2)"
+		h = "(height - 2)"
 		name = "buttons/button_normal/{BACKGROUND_IMAGE}.png{IPF}"
 	[/image]