Revamp main menu GUI and add lensflare

Large UI overhaul: refactors mainmenu and loading GUIs to use virtual screen expansion, multiple background layers, glitch/scanline effects, animated background transitions, and simplified intro video flow (named intro events). Adds lensflare UI controls and new lensflare vertex/fragment shaders (content/baseoq4/glprogs). Minor workflow/docs updates: add --source-root to manual-release workflow and add release notes draft. Miscellaneous small updates across renderer/framework/source files to support these GUI/renderer changes.

Author: themuffinator

.github/workflows/manual-release.yml

@@ -468,6 +468,7 @@ jobs:
             --release-scale "${{ needs.metadata.outputs.release_scale }}" \
             --release-reason "${{ needs.metadata.outputs.release_reason }}" \
             --repo "${GITHUB_REPOSITORY}" \
+            --source-root "${GITHUB_WORKSPACE}" \
             --run-id "${GITHUB_RUN_ID}" \
             --run-url "${GITHUB_SERVER_URL}/${GITHUB_REPOSITORY}/actions/runs/${GITHUB_RUN_ID}" \
             --output "${RUNNER_TEMP}/release-changelog.md"

.install/baseoq4/guis/loading/mplevel.gui

@@ -236,6 +236,7 @@ windowDef Desktop
 		background	"gfx/guis/common/gradient3"
 		matcolor	0,0,0,.5
 		matscalex	-1
+		screenalignx	right
 		screenaligny	top
 	}
 	windowDef t_message