custom shaders: add colorscheme information to shader uniforms

[ClearAspect]

#9417 Adds palette and color scheme uniforms to custom shaders, allowing custom shaders to access terminal color information:

• iPalette[256]: Full 256-color terminal palette (RGB)
• iBackgroundColor, iForegroundColor: Terminal colors (RGB)
• iCursorColor, iCursorText: Cursor colors (RGB)
• iSelectionBackgroundColor, iSelectionForegroundColor: Selection colors (RGB)

Colors are normalized to [0.0, 1.0] range and update when the palette changes via OSC sequences or configuration changes. The palette_dirty flag tracks when colors need to be refreshed, initialized to true to ensure correct colors on new surfaces.