Enlarged OS X invisible cursor.

Fixes #206.
Added GLFW 2 particles example.
Plain port of particles example to GLFW 3.
Formatted particles example.
Replaced ad-hoc argument processing with getopt.
Cleaned up particles example.
Fixed uses of relative time for cnd_timedwait.
Added CLOCK_REALTIME to clock_gettime emulation.
Replaced ad-hoc extension loading with glad.
Removed unused bits of glad.
Added dependency section to readme.
Merge remote-tracking branch 'upstream/master'