Cleanup: points_default.glsl -> las_points.glsl

Also remove obsolete resources.qrc
Add size to PointFieldData

Also cleanup to make resize() a free function (given that PointFieldData
has public members).
Add glew to thirdparty for convenience on windows
Build embedded glew by default on windows

This is purely to reduce external dependencies which have to be gathered
on windows to make building less of a massive PITA.
Minor fixes to appease MSVC

* Kill off a bunch of spurious warnings

* Work around unsupported C99 struct initialization syntax

* Work around odd bug in MSVC implementation of std::unique_ptr move
  constructor, which happens when nesting inside another struct.
Make las input optional and move out of pointarray.cpp

* Builtin readers are now available for ply and text, so it now makes
  some kind of sense to build without las support at all.  Las is on by
  default.

* Some minor tweaks to allow the automatic externals build to be turned
  off properly.