Fix GPU resource safety in constructors/destructors (#386)

[RobLoach]

Fixes #386

Audited all GPU/audio resource classes per the raylib C++ GPU resource guidelines.

Changes

RenderTexture — guard Unload() with IsValid() check; call Unload() before reassigning from raw ::RenderTexture

Music — guard Unload() with IsValid() check; call Unload() before reassigning from raw ::Music

ModelAnimation — guard Unload() with keyframePoses != nullptr check to prevent calling into raylib with an empty/moved-from object

VrStereoConfig — delete copy constructor/assignment; add defaulted move constructor/assignment

Window — delete copy constructor/assignment (manages singleton global state)

AudioDevice — delete copy constructor/assignment (manages singleton global state)