custom shader: add color scheme information to uniforms

[benodiwal]

Add new shader uniforms to provide access to terminal color scheme:

• iPalette[256]: 256-color ANSI palette array
• iBackgroundColor: terminal background color
• iForegroundColor: terminal foreground color
• iCursorColor: pure cursor color (without opacity)

The new iCursorColor uniform provides the unmodified cursor color
from the colorscheme, while existing iCurrentCursorColor retains
the opacity-modified version. This enables shader authors to create
sophisticated theme-aware effects and replace cursors entirely.

Color uniforms are populated per-frame when terminal colors change,
including dynamic updates via OSC sequences. All uniforms use
normalized float values (0.0-1.0) with RGB components.

The tests in this PR were written primarily by Claude Code.

Fixes #9417

[qwerasd205]

Is this PR AI-assisted? It's okay if it is, we just ask that you disclose that fact if so. (See CONTRIBUTING.md.)

[benodiwal]

Tests are written by agent. Let me disclose this in description. Thanks for pointing out.

[ClearAspect]

Should we really be trying to update the palette every single frame for this? Maybe this would be better to do on a config reload or an OSC sequence? I suppose bg_color is updated every frame so maybe this isn't actually such a big deal.

Still seems kind of wasteful. Maybe some way of marking the colors as dirty would be a better approach

[ClearAspect]

#9656