Use 32-bit visual by default if available.

With this patch, we use 32-bit visuals per default whenever it is
available. Otherwise, we fall back to the actual root window's
depth, which will typically be 24-bit.

Before this patch, we already used 32-bit depth for containers with
a window that uses 32-bit. However, this means that we didn't use
32-bit for split parent containers on which decoration is drawn.
For 32-bit windows using transparency, this caused a graphical glitch
because the decoration pixmap behind it would show through. This
behavior is fixed with this change.

relates to #1278

include/libi3.h

@@ -499,11 +499,14 @@ typedef struct color_t {
     double red;
     double green;
     double blue;
+    double alpha;
 
     /* The colorpixel we use for direct XCB calls. */
     uint32_t colorpixel;
 } color_t;
 
+#define COLOR_TRANSPARENT ((color_t){.red = 0.0, .green = 0.0, .blue = 0.0, .colorpixel = 0})
+
 /* A wrapper grouping an XCB drawable and both a graphics context
  * and the corresponding cairo objects representing it. */
 typedef struct surface_t {