Add visual tests for blendMode() across renderers

[avinxshKD]

Added BlendModeTest covering all 10 blend modes (BLEND, ADD, SUBTRACT, MULTIPLY, SCREEN, DARKEST, LIGHTEST, DIFFERENCE, EXCLUSION, REPLACE) plus one test that switches modes mid-sketch. Using JAVA2D for now since per-renderer baseline support isn't figured out yet. Windows baselines included, all 11 tests pass locally

Closes #1434

[avinxshKD]

Hey @mingness, PR is up

BlendModeTest covers the 10 modes + one that switches mid-sketch to make sure nothing leaks between mode changes. Using JAVA2D, baselines are Windows. Tests green locally.

[catilac]

@Vaivaswat2244 may I ask you to review this one? :)

[Vaivaswat2244]

Hey @avinxshKD, thanks a lot for your work on this.
PR looks good to me. I did leave a comment around adding some clarifying notes in the code — nothing that needs changing before merge, just things that'll make it easier to follow for whoever touches it next. Let us know if you need any help.

[avinxshKD]

Hey @catilac @Vaivaswat2244 added the comments, kept it minimal, just a one-liner on the helper explaining the test scene setup, a note on why BLEND gets called before the actual mode under test (that one was genuinely non-obvious), and a line in testModeSwitch saying what it's checking for. nothing structural changed

lmk if any changes needed

[avinxshKD]

Hey @catilac once I have my GSoC proposal ready would you be okay with taking a look at it?