wrlib: add explicit type definition in API to allow compiler Type Checks (3/3)

When defining enums as types instead of simple enums allows to use these
types at the places where the corresponding enum values are expected, then
allowing the compiler to check that, potentially reporting incorrect use
of values to the user.

This patch adds the type for the gradient style for RRender*Gradient.

Signed-off-by: Christophe CURIS <christophe.curis@free.fr>

wrlib/wraster.h

@@ -254,11 +254,11 @@ enum {
 /* 2 pixel width */
 #define RBEV_RAISED3	3
 
-enum {
+typedef enum {
     RHorizontalGradient = 2,
     RVerticalGradient = 3,
     RDiagonalGradient = 4
-};
+} RGradientStyle;
 /* for backwards compatibility */
 #define RGRD_HORIZONTAL  RHorizontalGradient
 #define RGRD_VERTICAL 	RVerticalGradient
@@ -426,11 +426,11 @@ void RFillImage(RImage *image, const RColor *color);
 void RBevelImage(RImage *image, int bevel_type);
 
 RImage *RRenderGradient(unsigned width, unsigned height, const RColor *from,
-                        const RColor *to, int style);
+                        const RColor *to, RGradientStyle style);
 
 
 RImage *RRenderMultiGradient(unsigned width, unsigned height, RColor **colors,
-                             int style);
+                             RGradientStyle style);
 
 
 RImage *RRenderInterwovenGradient(unsigned width, unsigned height,