add premultiplied alpha blend mode (#2342)

src/raylib.h

@@ -839,6 +839,7 @@ typedef enum {
     BLEND_MULTIPLIED,               // Blend textures multiplying colors
     BLEND_ADD_COLORS,               // Blend textures adding colors (alternative)
     BLEND_SUBTRACT_COLORS,          // Blend textures subtracting colors (alternative)
+    BLEND_ALPHA_PREMUL,             // Blend premultiplied textures considering alpha
     BLEND_CUSTOM                    // Blend textures using custom src/dst factors (use rlSetBlendMode())
 } BlendMode;