Add transparent MOAP background and click pass-through support

[shadoskill]

Description

This allows for media-on-a-prim to have its background fully transparent if the website sets it via CSS.
This also allow click pass-through to in world objects if clicking a fully transparent area on the moap.

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Additional Notes

Video of the feature and link to basic test page used in the video.

https://fullarray-dev.emptylist.co/moap_test.html

PowerToys_QwooSaIhZr.mp4

[github-actions]

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[shadoskill]

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[Copilot]

Pull request overview

Enables alpha-aware rendering for Media-On-A-Prim (MOAP) so web content can display with a truly transparent background, and updates picking/click handling so mouse events can pass through fully transparent media pixels to in-world objects behind.

Changes:

• Treat RGBA media textures as alpha in the render pipeline so transparency is respected.
• Add per-pixel transparency sampling for media textures (mask/pick support) via LLViewerMediaImpl::isTransparentAt().
• Update pie tool click/hover handling to defer to the visible pick when the media pixel is fully transparent; update CEF plugin defaults for RGBA/transparent background.

Reviewed changes

Copilot reviewed 8 out of 8 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
indra/newview/pipeline.cpp	Routes RGBA textures (including media) into alpha rendering when appropriate.
indra/newview/llviewertexture.h	Declares media-texture pick mask override to support transparency-aware picking.
indra/newview/llviewertexture.cpp	Implements media-texture getMask() by sampling the live media buffer.
indra/newview/llviewermedia.h	Adds isTransparentAt() API for per-pixel alpha testing at UV coords.
indra/newview/llviewermedia.cpp	Implements per-pixel alpha sampling and clears media placeholder with transparent alpha.
indra/newview/lltoolpie.cpp	Adds click/hover pass-through logic when the hit media pixel is fully transparent.
indra/media_plugins/cef/media_plugin_cef.cpp	Switches embedded CEF background/format to preserve alpha (RGBA/transparent).
indra/llrender/llgltexture.h	Makes getMask() virtual to allow media textures to override pick masking.

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[akleshchev]

This is likely to affect existing content, may be even render some content unreadable due to matching color of the background. I wonder if we can get some form of 'respect transparent background' checkbox to work on per-face basis. It should be trivial viewer side to store the value in LLMediaEntry, but server might will need to support it.