Image: Add improved blending method and test it.

core/image/common.odin

@@ -42,6 +42,23 @@ GA_Pixel      :: [2]u8
 G_Pixel_16    :: [1]u16
 GA_Pixel_16   :: [2]u16
 
+blend_single_channel :: #force_inline proc(fg, alpha, bg: $T) -> (res: T) where T == u8 || T == u16 {
+	MAX :: 256 when T == u8 else 65536
+
+	c := u32(fg) * (MAX - u32(alpha)) + u32(bg) * (1 + u32(alpha))
+	return T(c & (MAX - 1))
+}
+
+blend_pixel :: #force_inline proc(fg: [$N]$T, alpha: T, bg: [N]T) -> (res: [N]T) where (T == u8 || T == u16), N >= 1 && N <= 4 {
+	MAX :: 256 when T == u8 else 65536
+
+	r_a := u32(fg) * (MAX - u32(alpha)) + u32(bg) * (1 + u32(alpha))
+
+	return
+}
+
+blend :: proc{blend_single_channel, blend_pixel}
+
 Image :: struct {
 	width:         int,
 	height:        int,