Manually cherry-picked the following changes:
8c8d16de vp9 -> vpx in names
75b57d39 VP9_ -> VPX_ in function names
761a7088 VP9_INTERP_EXTEND -> VPX_INTERP_EXTEND
4273a52c VP9->VPX in border pixel macros
03568c31 VP9_FRAME_MARKER -> VPX_FRAME_MARKER
2334f51d VP9->VPX in fdct function names
Change-Id: Icc18dbf4b416dd0fa21033b3e19ab8a47c893508
Separate the common coefficient constant into vpx_dsp/txfm_common.h.
Move the SSE2 macro definitions to vpx_dsp/x86/txfm_common_sse2.h.
This clears the use case of vp9_idct.h in vpx_dsp folder.
Change-Id: I319735a2abf42888e5080ac14cfbcde34be7b121