* Refactor texture caching in render.Engine:
* New interface method: NewTexture(filename string, image.Image)
* WASM immediately encodes the image to PNG and generates a JavaScript
`Image()` object to load it with a data URI and keep it in memory.
* SDL2 saves the bitmap to disk as it did before.
* WASM: deprecate the sessionStorage for holding image data. Session
storage methods panic if called. The image data is directly kept in
Go memory as a js.Value holding an Image().
* Shared Memory workaround: the level.Chunk.ToBitmap() function is where
chunk textures get cached, but it had no access to the render.Engine
used in the game. The `pkg/shmem` package holds global pointers to
common structures like the CurrentRenderEngine as a work-around.
* Also shmem.Flash() so Doodle can make its d.Flash() function
globally available, any sub-package can now flash text to the screen
regardless of source code location.
* JavaScript API for Doodads now has a global Flash() function
available.
* WASM: Handle window resize so Doodle can recompute its dimensions
instead of scaling/shrinking the view.
* Refactor the event system in the WASM render engine to serialize the
async JavaScript events into a channel, so that queued events are read
off serially in the main loop similar to SDL. This fixes keyboard
input issues, altho if you type really fast some input keys get lost.
* Debug mode: no longer enables the DebugOverlay (F3) by default, but
does now insert the current FPS counter into the window title bar.
* ui.Frame: set a default "mostly transparent" BG color so the frame
background doesn't render as white.
* Add the MenuScene which will house the game's main menus.
* The "New Level" menu is first to be added.
* UI lets you pick Page Type and Wallpaper using radio buttons.
* Page Type: Unbounded, Bounded (default), No Negative Space, Bordered
* Fix bugs in uix.Canvas to fully support all these page types.
