C API: add sk_xfermode.h, impl, test

include/c/sk_types.h

Index: include/c/sk_types.h
diff --git a/include/c/sk_types.h b/include/c/sk_types.h
index 17ba92972a35b8bd5a50a4354463f1b2c9c453d4..f517434764d16c9a6d6814b8503247c11a697198 100644
--- a/include/c/sk_types.h
+++ b/include/c/sk_types.h
@@ -108,6 +108,38 @@ typedef struct sk_picture_recorder_t sk_picture_recorder_t;
 typedef struct sk_shader_t sk_shader_t;
 typedef struct sk_surface_t sk_surface_t;
 
+typedef enum {
+    CLEAR_SK_XFERMODE_MODE,
+    SRC_SK_XFERMODE_MODE,
+    DST_SK_XFERMODE_MODE,
+    SRCOVER_SK_XFERMODE_MODE,
+    DSTOVER_SK_XFERMODE_MODE,
+    SRCIN_SK_XFERMODE_MODE,
+    DSTIN_SK_XFERMODE_MODE,
+    SRCOUT_SK_XFERMODE_MODE,
+    DSTOUT_SK_XFERMODE_MODE,
+    SRCATOP_SK_XFERMODE_MODE,
+    DSTATOP_SK_XFERMODE_MODE,
+    XOR_SK_XFERMODE_MODE,
+    PLUS_SK_XFERMODE_MODE,
+    MODULATE_SK_XFERMODE_MODE,
+    SCREEN_SK_XFERMODE_MODE,
+    OVERLAY_SK_XFERMODE_MODE,
+    DARKEN_SK_XFERMODE_MODE,
+    LIGHTEN_SK_XFERMODE_MODE,
+    COLORDODGE_SK_XFERMODE_MODE,
+    COLORBURN_SK_XFERMODE_MODE,
+    HARDLIGHT_SK_XFERMODE_MODE,
+    SOFTLIGHT_SK_XFERMODE_MODE,
+    DIFFERENCE_SK_XFERMODE_MODE,
+    EXCLUSION_SK_XFERMODE_MODE,
+    MULTIPLY_SK_XFERMODE_MODE,
+    HUE_SK_XFERMODE_MODE,
+    SATURATION_SK_XFERMODE_MODE,
+    COLOR_SK_XFERMODE_MODE,
+    LUMINOSITY_SK_XFERMODE_MODE,
+} sk_xfermode_mode_t;
+
 //////////////////////////////////////////////////////////////////////////////////////////
 
 #ifdef __cplusplus