Fix issue with converting surfaces to 8 bit surfaces with palettes

[Temmie3754]

This resolves issue pygame/pygame#2477 by checking if the new surface needs a palette and creating the default one for it if it does.

[yunline]

LGTM

[MyreMylar]

Is there are chance you are leaking memory here for the all white palette? The SDL functions for setting the surface palette are careful to destroy/free (the functions have recently been renamed) any existing palettes before assigning a new one.

It looks like here you might potentially be just replacing the all white palette with the default palette leaving a dangling allocation for the old white palette.

On a wider scale it might be cool if this conversion function actually generated an appropriate 8 bit palette for the image being converted using something like the Median cut algorithm. See here for one example. This would be beyond the scope of this PR though.

[Temmie3754]

Thanks for catching that, it does indeed look as if there is a memory leak, I'll work on fixing that.

[Starbuck5]

This should get a unit test to make sure the issue doesn't regress in the future.

[Temmie3754]

Added a unit test

[Starbuck5]

Thanks!