Add error check for pixelDensity in mask() by pnngocdoan · Pull Request #1107 · processing/processing4

Resolves: #1065

Changes:
Add an error check for the mask() function in PImage.java to ensure both pixelWidth and pixelHeight of both imgs are matched.

Tests:
When pixel sizes don't match due to density difference, it will show an error line like below

When density doesn't match but underlying pixel size matches: