WASM Texture Caching

* 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.

dev-assets/doodads/azulian/azulian.js

@@ -1,5 +1,5 @@
 function main() {
-	log.Info("Azulian '%s' initialized!", Self.Doodad.Title);
+	Flash("Azulian '%s' initialized!", Self.Doodad.Title);
 
 	var playerSpeed = 12;
 	var gravity = 4;