Issue #26 Support texture drop onto PBR materials

build.yaml: Build shared branches

We're currently building every single commit pushed to Github. This is
racking up $20k in build charges a month and is generally superfluous.

This changeset alters build triggers so that builds automatically run if
they are committed to a **shared branch**:

- `release/*` - A release stabilization branch
- `project/*` - A project viewer branch
- `main/*` - The default/stable branch

PR commits are also automatically built.

...need to build another commit? Developers can trigger one using a
manual workflow run.

Issue #24 GLTF material disappears from object playing media

Check texture type, if media is stopped, it won't be a MEDIA_TEXTURE

resizeScreenTexture works if gResizeScreenTexture is true or render
target was resized

Update enhancement.md

Update bug.yaml

Add a basic CONTRIBUTING.md

Issue templates

Provide basic instructions in-repository for contributing to Second
Life.

Dataserver's version has compression artifacts

UIImgInvisibleUUID doesn't exist
Default normal for material is 'null'

Update mikktspace, tinygltf, vulkan_gltf packages to GHA builds.

Co-authored-by: RunitaiLinden <davep@lindenlab.com>

And hide warning when it's already 0

SL-19555 Fix picker failing to highlight default material in inventory

following promotion of DRTVWR-596