x86/vp9lpf: faster P7..Q7 accesses.

Introduce 2 additional registers for stride3 and mstride3 to allow
direct accesses (lea drops).

3931 → 3827 decicycles in ff_vp9_loop_filter_v_16_16_ssse3

Also uses defines to clarify the code.
x86/vp9lpf: refactor v/h using common macros for P7 to Q7.
x86/vp9lpf: add a preload system in FILTER_UPDATE.

Allow some macro refactoring in filter14().
avcodec/libfdk-aacenc: change MODE_7_1_REAR_SURROUND to map to AV_CH_LAYOUT_7POINT1

This was suggested by Rodeo on IRC
<Rodeo> sorry, I meant MODE_7_1_REAR_SURROUND would probably be AV_CH_LAYOUT_7POINT1

Reviewed-by: Jean First <jeanfirst@gmail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
avcodec/libfdk-aacenc: change MODE_7_1_FRONT_CENTER to map to AV_CH_LAYOUT_7POINT1_WIDE_BACK

This was suggested by Rodeo on IRC
<Rodeo> for consistency with the rest, MODE_7_1_FRONT_CENTER would be AV_CH_LAYOUT_7POINT1_WIDE_BACK (since LS+RS is mapped to back channels in other modes)

Reviewed-by: Jean First <jeanfirst@gmail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>