If no header includes the prototype of a function, then it cannot be
used from outside the translation unit it was defined in. In that case,
it should be marked as `static`, in order to avoid possible ODR
problems, unnecessary exported symbols, and allow the compiler to better
optimize those.
If this warning triggers in a function defined in a header, `inline`
needs to be added, otherwise if the header is included in more than one
TU, it will fail to link with a duplicate definition error.
The reason this diff got so big is that Lagom-only code wasn't built
with this flag even in Serenity times.
Skia now uses GPU-accelerated painting on Linux if Vulkan is available.
Most of the performance gain is currently negated by reading the GPU
backend back into RAM to pass it to the Browser process. In the future,
this could be improved by sharing GPU-allocated memory across the
Browser and WebContent processes.