Add error check for pixelDensity in mask()

[pnngocdoan]

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:

[SableRaf]

Thanks @pnngocdoan for your contribution! ✨

@all-contributors please add @pnngocdoan for code

[allcontributors]

@SableRaf

I've put up a pull request to add @pnngocdoan! 🎉