input: Avoid notifying unchanged scroll and paint state

[jlucaso1]

Summary

An input invalidated its cached parent on every unrelated redraw. Both update_scroll_offset and TextElement::paint called notify unconditionally, so a window drawing video rebuilt the input's surrounding screen on each frame.

Compare the clamped scroll offset before notifying. During paint, keep geometry notifications when bounds or scroll extent change, but do not notify for an unchanged layout. Editing, scrolling and resize invalidations remain intact.

Verification

• Added both regression tests before the fix. Both failed. An edited input rebuilt its cached parent 60 times for 60 root notifications.
• Both regressions now pass, including clamped no-op offsets, real scrolling, a shrinking scroll range, editing and resizing.
• After merging current main, cargo test -p gpui-base --lib --locked passed 842 tests.
• cargo clippy -p gpui-base --lib --tests --locked -- -D warnings passed on the updated branch.
• The compatibility version based on 5cb0946 also passed all 731 base tests and 412 component tests.

Consumers still on the 0.5 API can test the identical fix through jlucaso1/gpui-component branch fix/input-scroll-notify-0.5 at 970c606, without adopting the 0.6 dependency changes in current main.

[walterlow]

I was about to make a PR for this similar issue i encountered while developing my app as well