graphics: Don't import Vulkan implicit layers that aren't enabled
This modifies the runtime setup code to skip Vulkan implicit layers
entirely (and their dependencies) if they do not actually appear to be
used, in order to remove the risk of importing an unused layer breaking
unrelated things. The existing implicit layer tests now also use the
enable_environment
and disable_environment
values to make sure that
the environment is read correctly.
Fixes steamrt/tasks#533.
Signed-off-by: Ryan Gonzalez ryan.gonzalez@collabora.com