Graphics contexts not working as expected

[AntonPetrov83]

In Playdate contexts keep not only target image but all settings like setColor, setLineWidth etc.

When you popContext you should roll back to original graphics settings.

Currently playbit stores only target image in the context and remarkably thinks that not providing an image is suspicious:

function module.pushContext(image)
  -- TODO: PD docs say image is optional, but not passing an image just results in drawing to last context?
  @@ASSERT(image, "Missing image parameter.")
...

[AntonPetrov83]

I am working on it.
Most likely this will depend on #80