hevc: Add hevc_get_pixel_4/8/12/16/24/32/48/64

Checkasm timings:
block size bitdepth  C       NEON
4           8 bit:    146.7   48.7
           10 bit:    146.7   52.7
8           8 bit:    430.3   84.4
           10 bit:    430.4  119.5
12          8 bit:    812.8  141.0
           10 bit:    812.8  195.0
16          8 bit:   1499.1  268.0
           10 bit:   1498.9  368.4
24          8 bit:   4394.2  574.8
           10 bit:   3696.3  804.8
32          8 bit:   5108.6  568.9
           10 bit:   4249.6  918.8
48          8 bit:  16819.6 2304.9
           10 bit:  13882.0 3178.5
64          8 bit:  13490.8 1799.5
           10 bit:  11018.5 2519.4

Signed-off-by: Martin Storsjö <martin@martin.st>
