Standalone md5 model support

[NHogweed]

Now, if there is a md5 model, but no mdl with the same name, the engine will use the former instead of returning an error. This ignores r_loadmd5models and r_md5models.

[vsonnier]

I @NHogweed, thanks for this. Now, the code around the selection of which model to load it getting too messy for my taste, especially with the goto addition :) I'm going to move the model selection to a separate function to be more explicit and readable.

No need to touble yourself further though, I'll take care of the rest from this.

[NHogweed]

OK. I'm keeping my code until that though, just for any case.

[vsonnier]

Hi @NHogweed finally I merged your modification as-is. I wanted to be pedantic at first and redo this, but hey, I'm lacking time and as we say home : "le mieux est l'ennemi du bien" or if you prefer here Practicality beats Purity :)

Seriously though, your change was not that big, and is still easy enough to follow.

Thanks for your contribution !