vp9/encoder: fix segfault on win32 using vs < 2015

vp9/encoder: fix segfault on win32 using vs < 2015

shift the bsse[] member of the macroblock struct to the front to avoid
an incorrect offset (0) to the upper half of bsse[0] which leads to a
negative resulting in a crash. restrict this to visual studio versions
before 2015 (the bug was observed with 2013, fixed in 2015) to avoid any
potential cache impact on other platforms.

https://connect.microsoft.com/VisualStudio/feedback/details/2396360/bad-structure-offset-in-32-bit-code

BUG=webm:1054

Change-Id: I40f68a1d421ccc503cc712192263bab4f7dde076