fix(gpui): clip rounded media to visible bounds

[aronNormal]

Summary

• preserve caller-defined image boxes when both axes are constrained
• add visible clip bounds for rounded image masking across Metal, WGSL, and HLSL
• keep cover sampling/crop behavior aligned for images and surfaces

Verification

• cargo fmt --all --check
• git diff --check
• cargo check -p gpui
• cargo test -p gpui elements::img::tests (2 passed)
• cargo test -p gpui cover (2 passed)
• production-equivalent Flowform client build and visual acceptance

Context

Ports the narrow visible-clip approach from Zed PR zed-industries#59300 / issue zed-industries#44339 into
the Flowform GPUI fork. This is the first PR in the dependency stack.