Appropriately convert colors with 0 alpha to UIColor.clearColor (#76)

* Return appropriate object of  for all colors with 0 alpha.

* Update PSPDFKitPlugin.m

Improve comments

Author: nishantdesai

PSPDFKitPlugin/PSPDFKitPlugin.m

@@ -186,6 +186,13 @@ - (UIColor *)colorWithString:(NSString *)string
         if ([components count] > 3) {
             alpha = [[components[3] stringByTrimmingCharactersInSet:[NSCharacterSet whitespaceCharacterSet]] floatValue];
         }
+
+        // Return clear color if the alpha of the value supplied is 0.
+        // We internally check for clearColor when saving colors for the last used color. See #20042
+        if (alpha == 0) {
+            return [UIColor clearColor];
+        }
+
         if ([components count] > 2) {
             NSString *red = [components[0] stringByTrimmingCharactersInSet:[NSCharacterSet whitespaceCharacterSet]];
             NSString *green = [components[1] stringByTrimmingCharactersInSet:[NSCharacterSet whitespaceCharacterSet]];