fix(agc): honor disabled color targets and their reset state

[TibixDev]

What

Exclude COLOR_INVALID attachments from the effective color write mask. Preserve depth-only rendering and the export indices of later color targets. Initialize all eight color-format registers to their disabled reset state.

#1139 handles inactive pixel shaders and #1178 adds pixel-input defaults; neither handles these color targets.

Tested

Fedora 44, Release: driver, graphics, and PM4 tests. Covers disabled and partially written targets, depth-only draws, holes between color exports, missing registers, and context reset. Contribution checks pass. Windows was not tested.

Checklist

• Current main; no duplicate open PR.
• One topic.
• No new code comments.
• Unsupported paths still throw; UI stubs are documented.
• No notes, logs, or images in the repository.
• No new third-party code.
• General behavior.
• Depends on: none.
• AI-assisted: yes.

[TibixDev]

Rebased on 58a1bd66. Kept both the merged CMASK extent function and disabled-format write-mask handling.

Fedora 44 Release: agc_driver_tests, agc_driver_graphics_tests, agc_driver_pm4_tests build; the targeted tests and contribution checks pass. Windows CI is pending.

[github-actions]

Other open pull requests touch the same code. Merging one will break the other, or both implement the same thing. If this one builds on them, list them in Depends on.

Pull request	Conflicting files (lines on main)	Same exports	Same new files
#1414	core/libs/prx/libSceAgcDriver/tests/Graphics.cpp L580

[adriadam10]

Checked build + full ctest on current main + this PR (442/442, agc_driver_graphics passes), CI green on the head. COLOR_INVALID targets no longer count in the write mask in the decode, precheck and rejection paths, later export indices are preserved (State.cpp:647 and the test at Graphics.cpp:627), and a missing register still throws. Merging.