* 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.
Noah
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