wasm-ui: replace SDL2 with emscripten APIs
Refactors the webassembly render window interactor to use emscripten's html5.h. It now uses emscripten_* APIs to create a window, process mouse, keyboard events and create timers.
This works in
- wasm32
- wasm64
- wasm32+pthread proxying
This partially works in
- wasm64 + pthread proxying:
- mouse/keyboard event handling throws type error.
- upstream issue filed at emscripten#21851, fix emscripten!21852 will make it to 3.1.60 release.
Edited by Jaswant Panchumarti (Kitware)