View scaling

Provides consistent scaling of tiles across screens of different pixel densities and orientations and reorganizes View class to clarify functionality. 
Yeah there are two such redundancies that I left for the sake of
readability, but probably better to just have less code.
On Dec 11, 2014 7:19 PM, "Varun" <notifications@github.com> wrote:

> In core/src/view/view.cpp
> <https://github.com/tangrams/tangram-es/pull/30#discussion-diff-21721055>:
>
> >      setZoom(m_zoom + _dz);
> > +    m_dirty = true;
>
> setZoom is also setting dirty to true. This is redundant here.
>
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> <https://github.com/tangrams/tangram-es/pull/30/files#r21721055>.
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