Fix plot fill for one-sided plots (tidbyt#361)

If all plot points are far negative, the plot widget draws its surface up to invThreshold even if that value is outside of the plot bounds. The problem started to become apparent with the change to the shared drawing context.

Limit the maximum number of frames for an app (tidbyt#366)

Apps can return an arbitrarily huge number of frames in their render
tree. When painting, we keep all frames in memory to assemble an
animated WebP or GIF, so this can get out of control.

Instead, set a default maximum of 2000 frames per app. Any frames past
the 2000th frame will be discarded.

Limit number of parallel frames being painted (tidbyt#365)

In tidbyt#67, we started painting frames in parallel instead of serially, in
order to fully utilize available CPU.

But we did this without any limit. So if a `Root` had hundreds of frames,
we would attempt to paint all of them in parallel. This would consume
huge amounts of memory by allocating hundreds or thousands of image
buffers.

Instead, we now limit parallelism to the number of CPU's.

deps: Formally replace gg. (tidbyt#359)

This commit replaces the references in our codebase to import our
version of gg. The risk of the previous method is other modules don't
also make the override.

handle gif disposal methods (tidbyt#346)

perf: Fork fogleman/gg. (tidbyt#352)

This commit updates folgeman/gg to use our own fork that includes
performance enhancements.

deps: Track trunk for starlib. (tidbyt#337)

This commit bumps starlib to the main branch to pull in this change: qri-io/starlib#170

docs: Icon updates (tidbyt#334)

* icons: Update documentation for icons.

This commit updates the schema documentation for icons to match the icon
validation in the community repo.

* icons: Update icon usage.

This commit updates all examples to use icons that will validate in the
community repo.

fix: Remove quotes from qrcode. (tidbyt#316)

This commit fixes a bug where the URL contains quotes.

modules: Add QRCode module. (tidbyt#315)

* modules: Add QRCode module.

This commit adds a module to generate QR codes to pixlet! This native QR
code integration can be used to create promotional apps.

* Updated example

* Fix random seeding

* Address PR feedback