vp10: reimplement d45/4x4 to match vp8 instead of vp9.

This is more a proof of concept than anything else. The problem here
isn't so much how to code it, but rather where to place the resulting
code. All intrapred DSP code lives in vpx_dsp, so do we want the vp10
specific intra pred functions to live there, or in vp10/?

See issue 1015.

Change-Id: I675f7badcc8e18fd99a9553910ecf3ddf81f0a05