Fix compatibility with rgbds v0.6.x

[dannye]

rgbds v0.6.0 includes a rewrite of rgbgfx which breaks many of the pngs in this repo, for example https://github.com/pret/poketcg/blob/master/src/gfx/hand_cards.png

My first hunch is that the new rgbgfx now ignores all the embedded rgbgfx flags in the png files which we used to use, although I attempted to remove them in 652ce4e and 786574f but it's possible that I missed some.
It could also just be a matter of different default options.

For now, poketcg needs to be built with rgbds v0.5.x.

[dannye]

gbdev/rgbds#1117