sse2: add src_x888_0565

sse2: add src_x888_0565

Port of 2ddd1c498b to SSE2.

Uses the pmadd technique described in
http://software.intel.com/sites/landingpage/legacy/mmx/MMX_App_24-16_Bit_Conversion.pdf

Works around lack of packusdw instruction by first sign extending the
values.

fast:	src_8888_0565 =  L1: 681.40  L2: 689.20  M: 644.76 ( 25.51%)  HT:404.42  VT:288.04  R:306.07  RT:150.80 (1619Kops/s)
mmx:	src_8888_0565 =  L1:2056.03  L2:1985.44  M:1574.91 ( 61.87%)  HT:533.10  VT:376.35  R:416.10  RT:178.79 (1833Kops/s)
sse2:	src_8888_0565 =  L1:3793.42  L2:3653.44  M:1878.83 ( 73.94%)  HT:535.03  VT:407.96  R:421.46  RT:163.31 (1727Kops/s)

and for reference, using packusdw
sse4:	src_8888_0565 =  L1:4396.18  L2:4229.25  M:1904.04 ( 75.18%)  HT:559.79  VT:427.96  R:440.06  RT:165.71 (1744Kops/s)

Notice that MMX is faster in the RT case because it can operate on
8-bytes instead of the current 16-bytes for SSE2.