Improvement: --resize-damage

- Add --resize-damage to enlarge/shrink repaint region by a specific
  number of pixels, used for solving the line corruption issue with
  blur. Thanks to Nuck and jerri in #104 for reporting.

- Fix the memory leak of blur shader string.

src/common.h

@@ -403,6 +403,8 @@ typedef struct {
   /// Whether to paint on X Composite overlay window instead of root
   /// window.
   bool paint_on_overlay;
+  /// Resize damage for a specific number of pixels.
+  int resize_damage;
   /// Whether to unredirect all windows if a full-screen opaque window
   /// is detected.
   bool unredir_if_possible;