Correct OpenGL logic errors and enable debugging context

An error setting uniform values has been corrected.

Debugging logic has been added to trace logic errors; this
will be removed once testing has shown the code to work on
multiple systems.  These are manually inserted checks, rather
than using the (OpenGL 4) logging facilities.

If BIOFORMATS_OPENGL_DEBUG is defined at compile time, a
debugging context will be created.  However, because this
requires either an OpenGL extension provided by the driver
or OpenGL 4, this won't replace the manual checks above
unless the driver supports it.