Add some tests for render/view.

The single cursor tests are failing, probably due to a bug in text.Region.Intersection.
ViewRegionMap still needs testing.
Change the render/view tests.

0 size regions should be excluded when culling.
Always exclude 0 size regions in ViewRegions.Cull.
Merge pull request #356 from erbridge/render_view_test

Add some tests for render/view
backend/render: document view.go
Fix formatting.