Default jpeg quality = 97.

Flip Y when encoding PNGs.
Always free the snapshot buffer.

Use hack-y MINGW32 test to see which version of GLEW library to use --
"-lGLEW" on most systems, "-lglew32.dll" (libglew32.dll.a) on MinGW.

   - four digits by default
   - use .png suffix if libpng available, or if unix (assumes "convert" available)
   - use .ppm suffix if WIN32 and no libpng

for comparison with -c 'early commands', before window setup.

Add -tilt option to subcam command.

"vec"/"vecscale" control commands, and "vecvar" data command.

"vec off", "vec on", or "vec arrow".  With "arrow",
draws an arrowhead on each vector, with 3-D size equal to
the vector's length times the 'arrowscale'.  The arrowhead always
lies in the screen plane, so its size gives a cue to the vector's
true 3-D length (e.g. when the vector is viewed nearly end-on,
even a small arrowhead can look longer than the vector does).
This also means that arrowheads flip orientation when a vector
passes through being seen nearly end-on.

Arrowhead size is set by a new second parameter to "vecscale".
Default is 0.25, meaning that arrowheads are a quarter the main vector's length.

should we draw arrows too?  How large should arrowheads be (in 3D)
as a fraction of vector length?

Add checking for GLEW library, and --with-glew option.

Use glshader.h, and initialize shader stuff (once pre- and again post-window creation).

Not in use yet.

Add "vec/vectors" and "vecscale" control commands and "vecvar" data command.
When enabled, draws a vector at each point, as specified by some
triple of attributes given by "vecvar".  Length is [vecvar .. vecvar+2]
scaled by vecscale (default 1).

Add data for representing traj files (not yet used either).
Add data for drawing vectors at each point (this one is implemented now).

if we're using multiple source datasteps (in which case tfrac = realtime mod 1.0).
Check that the original realtime matches too.

of main window.  Apparently this used to be necessary to trigger initial redraw,
but (in more recent versions of FLTK?) just causes the main window to become
non-resizable.  Toss it.

Thanks to Jonathan Strawn <jonnyflash@gmail.com>,
UNM ARTS Lab, for figuring this out.