* In WASM build, user levels and doodads are written to localStorage
using their userdir path as keys (".config/levels/test.level")
* LoadFile() and WriteFile() for both Levels and Doodads interact with
the localStorage for WASM build instead of filesystem for desktop.
* userdir.ListLevels() and ListDoodads() for WASM scan the localStorage
keys for file names.
* userdir.ResolvePath() now works for WASM (previously was dummied out),
checks for the file in localStorage.
* Add RGBA color blending support in WASM build.
* Initial texture caching API for Canvas renderer engine. The WASM build
writes the chunk caches as a "data:image/png" base64 URL on the
browser's sessionStorage, for access to copy into the Canvas.
* Separated the ClickEvent from the MouseEvent (motion) in the WASM
event queue system, to allow clicking and dragging.
* Added the EscapeKey handler, which will abruptly terminate the WASM
application, same as it kills the window in the desktop build.
* Optimization fix: I discovered that if the user clicks and holds over
a single pixel when drawing a level, repeated Set() operations were
firing meaning multiple cache invalidations. Not noticeable on PC but
on WebAssembly it crippled the browser. Now if the cursor isn't moving
it doesn't do anything.
