Write more viewport and scissor tests

[greggman]

I need to write more scissor and viewport tests.

Specifically,

• that neither scissor nor viewport change when anything about the current
  framebuffer changes in including adding/removeing attachments, deleting
  attachments, changing the size of attachments, changing which fbo
  is bound. Some of these tests exist.
• that scissor can be anything that is not a negative width or negative height.
  In other words

scissor(-100000, -20000, 300000, 400000)

is valid

• That viewport can be set larger than the current backbuffer / fbo and that larger
  setting correctly effects rendering.