Proposed addition/replacement for transparent PNG test

Hi Wilhelm,

I have added at the bottom of our current mobile acid test a proposed
addition, that tests PNG transparency more fully than the current test,
in so that it uses alpha-channel transparency (which at least used to be
problematic in IE).
http://dev.w3.org/2008/mobile-test/test.html

We can either add it to the list of tests or use it to replace the
current "transparent PNG" test; let me know what you think.

Dom