implement enemy rendering.
add z-buffering and actual data (vs test data) drawing to the frame buffer to _drawenemies.
add unit tests to figure out how long enemy drawing our one enemy actually takes so we aren't guessing.
modify momentum0 to allow players to warp off the end of the level.  just one level, on repeat, for now.
fix a bug at momentum9 where it was trying to do momentum for a 3rd body and trashing memory causing the player to start falling through the bottom of the world well above the floor
and then come out some other dumb non-ceiling lcoation.
drawenemies had the sense of deltay backwards which was causing enemies to fall in from the floor.  I spent way too much time investigating that one.
drawenemies now uses X and Y in the inner loop, with X being the distance away/line length according to arctan and Y being the scanline we're currently plotting to.
drawenemies no longer attempts to loop over enemies to draw and just hard-codes the first one.
set an enemy location.
reset positions the enemy and sets its color.
add a new time to startofframe (a really bad place but it would be really hard to go anywhere else; at first, I played with rendering the enemies first but since their
lower five bits don't include their depth, they just get clobbered; we can z-buffer them over platforms but not platforms over them; at least not very well; they have
2 bits of depth data versus 5 for platforms)
timing was off in the 'enemy' path; an op I thought was wasting 3 cycles was only wasting 2.
clean up more in 'renderdone'.
platlevelresume goes away now.  besides, we were heavily re-using platform rendering state bytes needed to make that work.
new 'enemybird' graphics including an alternate animation frame not being used yet.
unit tests in 11_drawenemies.pl for rendering enemies.
02_arctan.pl:  remove end of framebuffer address hardcode and use the symbol instead.
remove the score display/num0 code, and fix a bug in _drawplayer where if it tried to draw more than 8 total lines of player data, it would just wrap around (which makes sense given that the line of player data to draw is encoded in a 3 bit field)
add some more platforms, just for kicks
add an INTIM check before _plotonscreen to help contain screen rolling
added a _maybe_vsync to check INTIM and do _vsync if it is zero
rework collisions to not loop but instead accept an index of which movable body to do collisions on
break out player specific collision stuff (killing the joystick button latch) so we can add monster stuff:  if the monster isn't colliding and the tick has a certain number of tailing zeros,
flap if the player is higher than we are.
tuning is messed up here.  flap constants used for the player don't work for the enemy.  even flapping very frequently, we have to use much larger flap values to get the computer moving.
collisions is now a subroutine-ish thing instead of a macro.
add the run_cpu with source code tracing to symbols.pm.