ws2812 -> pixbuf split

This is work in progress towards
nodemcu#2916

While here, document the ws2812 UART-based overlapping with mainline
execution.  Fixes
nodemcu#3140

This includes some untested hackery to keep the deprecated
ws2812_effects compiling.  We anticipate this module going away soon,
however.  See nodemcu#3122

Author: nwf

app/include/user_modules.h

@@ -44,6 +44,7 @@
 //#define LUA_USE_MODULES_PCM
 //#define LUA_USE_MODULES_PERF
 //#define LUA_USE_MODULES_PIPE
+//#define LUA_USE_MODULES_PIXBUF
 //#define LUA_USE_MODULES_PWM
 //#define LUA_USE_MODULES_PWM2
 //#define LUA_USE_MODULES_RFSWITCH