fix dynamic bpp-target when checking for supported video modes

[sezero]

@icculus, @slouken: Comments?

[icculus]

I don't understand why this is being changed from 7 (the last element in the array) to 6.

I'm sure this is something obvious, but an explanation would be helpful here.

[pionere]

The [6] element (second last element) is supposed to be the place for the dynamic bpp. The [7] element (last element) is just a 'terminating' entry which must be zero, otherwise the loop might never end. The reason it worked till now is that the [6] element is never changed and stopped the loop prematurely.

[sezero]

@icculus ?