Use PDAL instead of liblas

[a17r]

Taken from (with minimal squashing): openscenegraph#1093

[AnyOldName3]

Two things immediately come to mind:

• PDAL is a library built with CMake which properly exports its targets, so should be imported using its config files with find_package in CONFIG mode. It shouldn't be using a finder module as those are only meant for non-CMake libraries or libraries that haven't yet started exporting their targets properly.
• OpenMW does not use point clouds, so this PR is entirely out of scope for this fork.

[a17r]

@AnyOldName3 thanks for your reply even if the LAS module is out of scope. My motivation here is that this fork is still close enough to upstream's 3.6 branch, while upstream's seems to be abandoned. As distributions are struggling to keep OSG going, they may be starting to look at other sources like this fork. And laszip is one of these dependencies that is dead, but thankfully has a replacement with PDAL.

[AnyOldName3]

This fork exists for OpenMW-specific improvements that are expected to break other apps. It's not appropriate to use this for non-OpenMW OSG-based projects unless their developers have vetted our changes and confirmed that their apps are compatible.