Canvas Backend Blend Support (Issue #165)

[Atharva-Kanherkar]

This PR adds support for the Blend algebra to the Canvas backend.

Implementation:

1. Canvas Blend Implementation:

   • Added CanvasBlend trait in canvas/src/main/scala/doodle/canvas/algebra/CanvasBlend.scala
   • Implemented all methods required by the Blend trait using Canvas's globalCompositeOperation
   • Used a private blend helper function to avoid code duplication

2. How it works:

   • Each blend method uses the Finalized and Renderable pattern to wrap the original drawing operation
   • The wrapper saves the canvas context, sets the appropriate blend mode, runs the original drawing, then restores the context
   • This preserves all other drawing state while applying the blend effect

Testing:

I noted that the golden test infrastructure uses Java2D (which doesn't support the Blend trait), so I haven't included automated tests.

Based on the discussion in the issue, I understand that:

1. Automated visual testing is only available for Java2D
2. Canvas blend modes should be tested through example pages in documentation
3. Adding Blend to Java2D would be a separate issue (Implement blending modes for Java2D #191)

I'd appreciate feedback on this implementation, particularly on:

1. The approach used to wrap the original drawing functions
2. Suggestions for appropriate testing strategies

[Atharva-Kanherkar]

hi @noelwelsh , would love to receive your feedback on this!

[noelwelsh]

A few small changes to follow the existing conventions.

[Atharva-Kanherkar]

Hey @noelwelsh , I have added the requested changes!

[noelwelsh]

A few small changes.

[noelwelsh]

saving and restoring the canvas state is quite expensive (there is a lot of state). Better to just save the current blend mode (i.e. the value of globalCompositeOperation).

[Atharva-Kanherkar]

@noelwelsh sure ill get back to you!