better @2x / @4x support

bmf2 is now correctly reading all of the different assets, with the
right suffixes. As shown in the example, there is 3 resolutions: normal
@2x and @4x. depending on the device, bmf2 will load and read the
correct .lua files. be sure to export all your targets for ‘corona 2.0’
as shown in the .bmglyph
new lua file