Fix Liquify Accurate with Instant Preview

1. Fix Instant Preview Mode not working on zoomed out canvas in Liquify. https://bugs.kde.org/show_bug.cgi?id=514407
2. Fix gaps on the edges of rectangular selection when going out of the selection with Liquify. https://bugs.kde.org/show_bug.cgi?id=514406
3. Add debug painting for PainterPolygonOp
4. Add a safety check for pixelPrecision in transformSrcAndDst

Test Plan

1. Make a big canvas, zoom out, and start Liquify in Accurate with Instant Preview mode. Use Liquify. The preview should work.
2. Paint something, make a rectangular selection. Use Liquify, make sure to go outside of the selection (especially on the bottom and the right side). Apply. There should be no gaps or leftover pixels on any of the sides. (Note: in Instant Preview mode, when zoomed out, some gaps are possible due to calculation issues, but they disappear when you apply, so it's fine).
3. If you're building from source, you can enable DEBUG_PAINTING_POLYGONS in kis_grid_interpolation_tools.h, then do the same as above. All the polygons should look correct, there should be no weird gaps or sticking out pieces. The edges of the dark red polygons should be regular.

Formalities Checklist

• I confirmed this builds.
• I confirmed Krita ran and the relevant functions work.
• I tested the relevant unit tests and can confirm they are not broken. (If not possible, don't hesitate to ask for help!)
• I made sure my commits build individually and have good descriptions as per KDE guidelines.
• I made sure my code conforms to the standards set in the HACKING file.
• I can confirm the code is licensed and attributed appropriately, and that unattributed code is mine, as per KDE Licensing Policy.
• Does the patch add a user-visible feature? If yes, is there a documentation MR ready for it at Krita Documentation Repository?

Reminder: the reviewer is responsible for merging the patch, this is to ensure at the least two people can build the patch. In case a patch breaks the build, both the author and the reviewer should be contacted to fix the build. If this is not possible, the commits shall be reverted, and a notification with the reasoning and any relevant logs shall be sent to the mailing list, kimageshop@kde.org.