Fix math and shift overflow in function visual_color_to_uint32

Clang said:
> [..]/libvisual/lv_color.c:263:16: error: signed shift result (0x100000000) requires 34 bits to represent, but 'int' only has 32 bits [-Werror,-Wshift-overflow]
>         colors = (256 << 24) |
>                   ~~~ ^  ~~

A closer look reveals that 255 (0xFF) is the maximum alpha value
and that 256 (0x100) should have been 255 in the first place.

Original report at https://sourceforge.net/p/libvisual/bugs/18/

Author: hartwork

libvisual/libvisual/lv_color.c

@@ -260,7 +260,7 @@ uint32_t visual_color_to_uint32 (VisColor *color)
 
 	visual_log_return_val_if_fail (color != NULL, 0);
 
-	colors = (256 << 24) |
+	colors = (0xFF << 24) |
 		(color->r << 16) |
 		(color->g << 8) |
 		(color->b);