Some changes to the trunc

Hi,
I was trying to animate some png images with alpha when I encountered bad performance (I rewrote the same script in python and the performance was a lot better). I found out that I had to use SDL::DisplayFormatAlpha to fix the performance, but SDL::Surface::display_format_alpha was missing, so I added it (it's the same like SDL::Surface::display_format, but uses SDL::DisplayFormatAlpha).

The second change I did to truck was to change $SDL::DEBUG to a constant, so the compiler can optimize away the blocks with SDL::DEBUG.

Everything is tested and works.