Load shader files in filename order

In Quake 3, pak0.pk3 shader /file names/ have priority over all other shader /file names/.
You cannot add a new shader file that replaces one shader definition, because the pak0.pk3 /file names/ have priority.
You basically must replace the entire shader file that the shader definition is in.

Let's load shader files based on file name order so that pak1.pk3/scripts/gfx_mymod.shader and zz_mymod.shader can override shaders in pak0.pk3/scripts/gfx.shader.

Because this changes the order shader files are loaded in, it could cause compatibility issues with vanillia Q3.

Also, always sort FS file lists. (That's actually the only code changes made.)