bitmaps in vector form cause projects to run slowly when open in paint editor

[BryceLTaylor]

Expected Behavior

Projects should run quickly all of the time.

Actual Behavior

With a project running, if you open the paint editor and select a costume that has been converted from bitmap to vector in a scratch 2.0 project, the project will slow down tremendously.

Steps to Reproduce

I'm still working on repro steps to make a project that has this effect. There are two projects that I know of that have this behavior already. Space Giraffe (here it is in 2.0)and Geometry Dash (here it is in 2.0)

Load one of the above projects
Click the green flag
Open the Paint Editor
Select a the "background" sprite in Geometry Dash or the "cover" sprite in Space Giraffe
Note: the project runs incredibly slowly

Operating System and Browser

Chrome on Mac

[fsih]

From Z:

The space giraffe gui issue relates to the paint wrapper being rerendered every game frame and part of updating it is re-decoding the svg from binary into utf8. Most of the time is that re-decoding. We can make a quick change to scratch-storage to use browser-implemented TextDecoder and TextEncoder. Some quick testing in Chrome shows the browser-implemented TextDecoder is 10x faster than the JS implemented TextDecoder.

Probably also want to change the gui, to relax re-rendering scratch-paint if it isn’t making changes when the vm is actively running.

scratchfoundation/scratch-storage#44