expose capture stats via shm

[Denzy7]

the end intention is to allow mangohud to read capture data like file size and recording time, much like how msi afterburner does it
it had previously been a requested feature

issues:

multiple clients might reset record stats [ ❌ ]

• perhaps move the creation of shm to the plugin? but mangohud won't find it unless it's also signalled shm is ready for reading

[nowrep]

This should be a separate OBS plugin, as it has nothing to do with Vulkan/GL game capture.

[Denzy7]

oh great catch! it can be even compiled in mangohud source if libobs exists. thanks!