[rlgl] Use defines in default shaders

[floopfloopfloopfloopfloop]

The #ifndef RL_DEFAULT_SHADER_ATTRIB_NAME_POSITION et all in rlgl.h imply the names can be redefined, but the default shader sources use hardcoded names, resulting in name mismatches and nothing drawing if one attempts. This PR changes the shaders to use the defines, which seems to fix the issue.

[floopfloopfloopfloopfloop]

what in tarnation Apparently you can unwittingly use other people's emails in git commits, and github will just say it was made by them. Amazing.

[raysan5]

@floopfloopfloopfloopfloop this change reduces default shaders readability and actually makes me question if shader names should be allowed to be changed, nobody requested it in the last 12 years. Most probably not merging it but probably making some changes in consequence.

[floopfloopfloopfloopfloop]

12 years is a long time, I'm surprised. Readability could be easily fixed with temporarily redefining the attribute names?

#define RL_VERTEX_POSITION RL_DEFAULT_SHADER_ATTRIB_NAME_POSITION
//...
"attribute vec3 "RL_VERTEX_POSITION";     \n"
//...
#undef RL_VERTEX_POSITION

Being able to avoid mismatched variable name style conventions without forking is admittedly an unusual and rather aesthetic feature I was quite happy to see.