fixes #2619

After a call to get()/set(), pixels array has been treated as up-to-date
even though user has been drawing. Because of this, loadPixels() was not
updating pixels array with changes.
Moved pixels invalidation to endShape

Moved to account for a possibility that user uses get(), set() or
loadPixels() inside of beginShape()/endShape() block.
Changed setgetPixels to arePixelsUpToDate

Changed the name to better reflect the purpose of the variable.
Merge pull request #2638 from JakubValtar/filter-fix

fixes #2619 - filter() not applying to images produced by saveframe() consistently