[racket-dev] Caching rendered icons

The time taken to render icons is becoming noticeable when DrRacket 
starts up. I estimate ~300ms on my beefy laptop, and it will probably 
double. 20ms per icon is fine until there are 50 icons and icon parts.

(This also would have happened if the icons were rendered from SVG in 
Racket. The ominous-sounding ray tracer is actually quite fast, in part 
now because of Typed Racket's optimizer.)

Programs like DrRacket should load cached icons instead of rendering 
them at startup. I've had two ideas for this.


1. Runtime: Provide `bitmap-file-ref!'. The contract would be

     bitmap-file-ref! : path-string? any/c (-> bitmap%) -> bitmap%

It works like `hash-ref!', but the "hash" is a subdirectory name, the 
key is md5-ed to make a file name, and the thunk produces the icon.

Which directory would I put them in? Would I make a subdirectory of 
'pref-dir? (Not 'temp-dir, because that might be the current directory.)


2. Compile time: Provide macros that render icons during expansion and 
try to store them in the "compiled" directory. They expand to code that 
loads the icon if the file exists; renders it otherwise. This turns 
rendering into compiling and icons into program binaries, which Seems Right.

Is there a way to reliably get the "compiled" directory path during 
expansion, and then load files from it at runtime? Can I ensure that 
.PNG files are distributed automatically?

Neil T