Complete mask element implementation

[usiegj00]

Summary

This PR completes the partial mask element implementation that was started but not finished.

Changes

• Fixed the apply_mask method to properly work with Prawn's soft_mask API
• Properly structured soft_mask blocks with nested calls
• Added proper call context management using push/pop_call_position
• Process mask children to generate the mask pattern correctly

Problem

The previous partial implementation wasn't correctly handling the soft_mask API which expects drawing commands to be nested within the block. This caused masks to not render properly in PDFs.

Solution

The fix ensures masks work correctly by:

1. Using save_graphics_state for proper state management
2. Managing call contexts with push/pop_call_position
3. Processing mask children in the correct context
4. Properly nesting mask drawing commands within the soft_mask block

Testing

Tested with various mask types including:

• Circular masks for QR codes
• Rectangle masks with gradients
• Complex mask patterns

All tests pass and masks now render correctly in PDFs.

Fixes the incomplete mask implementation from commit 8fe7d2f.

[mogest]

Hi @usiegj00, thanks very much for finishing this off!

I noticed you've made this PR (#185) as well as #186, which look very similar. There are also quite a lot of merge conflicts. Would you be able to prepare a single PR for review? I'd be very happy to bring this work in.

[usiegj00]

Closing in favor of #186, which supersedes this PR. #186 includes the 1c394d2 soft_mask fix from here and adds complete mask support (including /image-based masks), resolving #148. Let's continue review there. Thanks!