Bug fix: Issue #40: -z does not work as expected

More information in the issue report.

- Let window opacity affect the opacity of its shadow and frames even if
  -z is enabled.

- Check for the range of -o to eliminate potential segfault.

src/compton.h

@@ -152,6 +152,23 @@ extern int root_height, root_width;
 // inline functions must be made static to compile correctly under clang:
 // http://clang.llvm.org/compatibility.html#inline
 
+/**
+ * Normalize an int value to a specific range.
+ *
+ * @param i int value to normalize
+ * @param min minimal value
+ * @param max maximum value
+ * @return normalized value
+ */
+static inline double normalize_i_range(int i, int min, int max) {
+  if (i > max)
+    return max;
+  if (i < min)
+    return min;
+
+  return i;
+}
+
 /**
  * Normalize a double value to 0.\ 0 - 1.\ 0.
  *