This implements the features from #44. That one could probably be closed and further discussion/work done here?

• glpCheckErrors(): XS-accelerated glGetErrors which automatically loops, warns individual error ids, and croaks if any occurred
• glpSetAutoCheckErrors(1|0): enables running of glpCheckErrors before and after every GL call, in XS

Problems so far:

• gluGetErrorString is missing, so the individual warns just dump the number
• i couldn't figure out how to wrap the glpCheckErrors code into a C function that could be called from all the gl calls, so right now it copies the block twice for every call, probably not great
• probably missed a bunch of potential problems

Observation from running it in Microidium:

• no noticable performance impact difference between OpenGL::Modern master and this branch
• no noticable performance impact difference from having the auto-checking on, which is a little strange, but it works