debugging: implement r_showNormalMaps and r_showMaterialMaps

[illwieckz]

implement r_showNormalMaps

r_showEntityNormals is dropped, in any way, vertexLighting_DBS_entity
and vertexLighting_DBS_world glsl are asking for merge

also beautify some code for other r_show* cvar

also introduce r_showMaterialMaps to debug specular and physical maps.

[illwieckz]

I was having problems with specular maps, so I added r_showMaterialMaps… This to discover a mistake I did… and to somwhat fix PBR in the process…

[illwieckz]

I would like to merge #256 before this one so I can also implement r_normalMapping in liquid shader.

[slipher]

A warning message if 2 or more of these mutually exclusive cvars are enabled would be nice. Otherwise looks good to merge.

[illwieckz]

rebased on master and implemented r_showNormalMaps on liquidmap_fp.glsl

[illwieckz]

@slipher A warning message if 2 or more of these mutually exclusive cvars are enabled would be nice. Otherwise looks good to merge.

I would like to have such feature but that seems not be very easy to do, as we may test all conflicting options…

A better implementation would be something like `r_showDebug normalmaps' or something like that, with a number for each type. We may create an issue for this but to me that looks out of topic of this PR.