Test skip on and off in RDO loop (#474)

* Refactor RDO loop to avoid code duplication

Use same code regardless of whether a loop over chroma modes
is needed

* Test skip on and off in RDO loop

* Don't search transform type when skipping

* Don't try to skip when using intra modes

* Fix issues with speed 0 and speed 1

Handling of < 8x8 was incorrect in CDEF and in reset_skip_context()

* Use dense sampling for chroma components in above_coeff_context and left_coeff_context