backend: allow transparent windows to clip other windows

Transparent windows usually blends on top of other windows, this commit
adds an option to make transparent windows clip other windows like
non-transparent windows do.

Closes #265

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

src/config.h

@@ -235,6 +235,10 @@ typedef struct options {
 
 	// Don't use EWMH to detect fullscreen applications
 	bool no_ewmh_fullscreen;
+
+	// Make transparent windows clip other windows, instead of blending on top of
+	// them
+	bool transparent_clipping;
 } options_t;
 
 extern const char *const BACKEND_STRS[NUM_BKEND + 1];