Read only header bytes when creating textures list

R_LoadTextures was reading in the entirety of every
texture lump, now it only reads the 8 byte PNG header.
This saved more than 1 second for me (1.68 s -> 0.24 s).

PNG still need to be read in entirely to check their
dimensions; I didn't bother looking into optimizing it
since we don't have many PNG textures right now.