* We store a Go image.Image copy of all textures in the SDL2 renderer.
* When the underlying SDL2 textures are freed, regenerate them when
needed from the underlying Go Image.
Since SDL2 is using in-memory bitmaps the same as Canvas engine, the
function names of the render.Engine interface have been cleaned up:
* NewTexture(filename, image) -> StoreTexture(name, image)
Create a new cached texture with a given name.
* NewBitmap(filename) -> LoadTexture(name)
Recall a stored texture with a given name.
* level.Chunk.ToBitmap uses simpler names for the textures instead of
userdir.CacheFilename file-like paths.
* 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.
