framebuffer: validate width/height in _set_viewport
framebuffer: validate width/height in _set_viewport

This validates that the viewport width and height arguments are positive
values in _cogl_framebuffer_set_viewport. In addition, just before
calling glViewport we also assert that something else hasn't gone amiss
and that the internal viewport width/height values we track are still
positive before passing to glViewport which generates an error for
negative values.

collects/cogl,spatulasnout/cogl,djdeath/cogl,gcampax/cogl,spatulasnout/cogl,djdeath/cogl-android,Distrotech/cogl,djdeath/cogl-android,djdeath/cogl,djdeath/cogl-android,Distrotech/cogl,collects/cogl,djdeath/cogl,gcampax/cogl,Distrotech/cogl,Distrotech/cogl,gcampax/cogl,gcampax/cogl