UI: Fix wrong order for sources_mutex

[walker-WSH]

Description

Below callstack is from graphic thread where gs_enter has been gotten. As this, the order of requesting mutex is as gs->sources_mutex. That is conflicting with basic logic in libobs. About basic request order, sources_mutex should be requested before gs.

[callstask]
render_displays
(1)request gs enter
DrawSpacingHelpers()
CreateLabel
obs_source_create_private
(2)request obs->data.sources_mutex

Motivation and Context

keep the stable order for requesting mutex. otherwise there will be a nested logic which will cause block.
But what I must declare is that this block is not easily happen

How Has This Been Tested?

run obs and select any video source,
ensure pixel label in preview window can be displayed normally

Types of changes

Bug fix (non-breaking change which fixes an issue) -

Checklist:

• My code has been run through clang-format.
• I have read the contributing document.
• My code is not on the master branch.
• The code has been tested.
• All commit messages are properly formatted and commits squashed where appropriate.
• I have included updates to all appropriate documentation.

[Lain-B]

Question: You say that it's in conflict with libobs logic. Where does the sources_mutex get locked before the graphics mutex in libobs?

Comment: An alternative way to handle this would be to exit the graphics mutex before the label creation, and then enter it again after. That should be safe to do as well, although that code might be equally awkward. (Nevermind, this alternative I made would be bad, ignore this part please)

[walker-WSH]

hi @Lain-B

(1) in obs_load_sources, sources_mutex will firstly be requested, then create all sources.
in obs_source_info::create function, gs may be requested since gs_texture need to be created in any plugins.

(2) in the past, tick_sources function will call tick function for all sources and gs may be requested in any tick function to update texture.
surely, now tick_sources has no this case since tick function is running outside of sources_mutex

[walker-WSH]

another method:

create label source early, not late in display callback