Fix: Empty > Image not working in some hardwares

Basically the default U.glalphaclip was 1.8367099231598242e-40 in some
computers (e.g., OSX 10.11, AMD Radeon HD 6750M) this value would need for bad
gl clipping.

The new default is 0.0. To test this before/after the fix, just run:

$ blender -b --factory-startup --python-expr "import bpy; print(bpy.context.user_preferences.system.gl_clip_alpha)"

source/blender/blenloader/intern/versioning_defaults.c

@@ -63,6 +63,9 @@ void BLO_update_defaults_userpref_blend(void)
 
 	/* default from T47064 */
 	U.audiorate = 48000;
+
+	/* for some reason U.glalphaclip was 1.8367099231598242e-40 */
+	U.glalphaclip = 0.0f;
 }
 
 /**