rain bug fix and new engine

doc/CHANGELOG.md

@@ -140,6 +140,18 @@ Versions follow `MAJOR.MINOR.PATCH-REVISION_STAGE_N` (e.g. `3.99.1-4_rc_1`).
   (they were evaluated as arithmetic expressions); `pwdscore` classifies
   characters with locale-proof classes; README default of
   `GENPWD_DEFAULT_OCCURS` corrected.
+- `rain.sh`: the terminal was left with the last drop colour active, the cursor
+  parked over the frozen last frame and the user's `INT`/`TERM`/`WINCH` traps
+  deleted and `stty` forced to `echo`: the screen is now cleared and the traps
+  and terminal settings restored; a resize now clears the screen and drops
+  (drops placed for the old size lingered); a drop vanished for one frame each
+  time another one left the screen; each frame is written with one `printf`
+  instead of one per character (output is byte-identical, a lot faster);
+  colour and charset names are case-insensitive (`RAIN_DEFAULT_COLOR=Green`, as
+  documented, fell back to white), an unknown colour is an error instead of a
+  silent white, a null speed no longer spins the CPU, a non-terminal stdin or
+  stdout is refused instead of looping, and the usage mentions `q` to quit;
+  README defaults of the `[rain]` keys corrected.
 - `compress.sh`: option flags and loop variables no longer leak into the
   interactive shell.