backend interface: add IMAGE_OP_RESIZE_TILE

We need this to paint the wallpaper with repeat to mimic the behavior of Xorg's
background pixmap.

Signed-off-by: Yuxuan Shui <yshuiv7@gmail.com>

src/backend/gl/gl_common.h

@@ -46,8 +46,9 @@ typedef struct gl_texture {
 	int *refcount;
 	GLuint texture;
 	GLenum target;
-	unsigned width;
-	unsigned height;
+	int width, height;
+	// The effective size of the texture
+	int ewidth, eheight;
 	unsigned depth;
 	bool y_inverted;
 	bool has_alpha;