some vfs improvements

1. refresh vfs before loading the map, in DPK scenario, it prevents the false-positive of unknown textures because VFS is not refreshed yet when the map is loaded
2. loads notex/shadernotex from a builtin VFS, simplify code by relying on common shader code to display them, allow games to override them (like smokinguns does)
3. fix the default texture: uses textures/radiant/notex path instead of the wrong textures/ path which is a directory path

Bug fixed in 3. was already there before but the fix in 1. made it obvious (that faulty path is listed in texture path when vfs is refreshed before map is loaded), and 2. is required to fix 3.

Basically, the whole PR fixes the faulty unknown texture messages (because of map/vfs loading order or because of wrong behavior in former code), simplifies the code by reusing existing parts instead of reinventing the wheel for the notex textures, allows custom notex textures.