wayland: Pre-multiply custom cursor image alpha

Since the Wayland SHM buffer format is implicitly premultiplied and the
GLFWimage pixels are defined to be non-premultiplied, we need to
convert the non-premultiplied pixels to premultiplied when filling the
buffer.

Related to #707.

src/internal.h

@@ -233,6 +233,13 @@ typedef VkResult (APIENTRY * PFN_vkEnumerateInstanceExtensionProperties)(const c
         y = t;                    \
     }
 
+// Helper for non-premultiplied alpha to premultiplied alpha conversion
+static inline unsigned char _glfwMultiplyAlpha(unsigned char alpha,
+					       unsigned char value)
+{
+    return (unsigned char) ((value * (unsigned int) alpha) / 255);
+}
+
 
 //========================================================================
 // Platform-independent structures