IMPACT rendering fixes

[iblowmymind]

On IMPACT, iconifying Console with Twilight active can turn X windows black, and File Manager/Icon Catalog scrolling changes shades and damages icons. Softimage|3D also shows a red-tinted splash, hides its popup dialogs, and clears the viewport backgrounds, grids, and compasses to black. These fixes preserve occluded X content and restore Softimage's and PowerAnimator's native drawing paths.

• Model clipping-ID planes and PP1 window clipping controls for GL root repaints.
• Use consistent expanded RGB444 colors for fills, stipples, fragments, and transfers. Decode read buffers independently, honor buffer B's masks, replicate colors to both buffers, and avoid applying logic operations twice to aliased pages.
• Convert GE RGBA uploads to the RSS transfer component order and preserve alpha, fixing the Softimage splash.
• Honor native absolute draw selectors, overlay format and plane masks, and both main buffers. This restores Softimage menus, glyphs, and dialogs while preserving main-plane content.
• Return the current color/index for GE state reads instead of echoing the requested address. Native IRIS GL clear() reads the current index; the old reply became zero and erased Softimage's background planes.
• Expose write masks, viewport and scissor bounds, clear state, matrix stacks and matrices through native libGLcore state queries.

Validation:

• All 110 IMPACT tests pass with JITv2 and HostGL, including all 4,096 RGB444 colors through repeated scroll round trips, buffer selection/masks/replication, RGBA uploads, native overlay clears/geometry/glyphs, and current-color readback.
• Live IRIX 6.5/High IMPACT: copied scroll regions are pixel-identical in File Manager (164,480 pixels) and Icon Catalog (62,715 pixels), with regular and Twilight backgrounds. Six full scroll cycles and a fresh repaint preserve content. Console map/iconify under Twilight preserves both windows.
• Live SI3D Extreme launch: corrected splash; gray backgrounds, grids, and XYZ compasses in all four viewports; visible nested primitive menus and cube dialog; cube creation; visible exit dialog and successful cancellation. Background and grid remain intact through these actions.
• Native IRIS GL probe returns selected indices 21, 31, and 19 correctly through getcolor().
• Release CLI and GUI builds pass with JITv2 and HostGL. git diff --check passes.

[techomancer]

reviewed, fixed and merged in #188