Support for integer shader attributes

Pass integer fields through to shader using glVertexAttribIPointer when
they are integers and not marked as fixed point numbers.

* Various cleanups in shaders, to remove extra scaling which isn't
  required any more.

* Move shader attribute location lookup out into glutils; include base
  type in ShaderAttribute.

* Mark intensity, returnNumber, numberOfReturns, and classification as
  non-scaled integer values.