Remove the unused FORMAT_COFF define

Nothing in the project currently sets FORMAT_COFF - the other generic
branch works just fine on windows.
Backup/restore the xmm6-xmm15 SSE registers within asm functions on win64

According to the Win64 ABI, these registers need to be preserved,
and compilers are allowed to rely on their content to stay
available - not only for float usage but for any usage, anywhere,
in the calling C++ code.

This adds a macro which pushes the clobbered registers onto the
stack if targeting win64 (and a matching one which restores them).
The parameter to the macro is the number of xmm registers used
(e.g. if using xmm0 - xmm7, the parameter is 8), or in other
words, the number of the highest xmm register used plus one.

This is similar to how the same issue is handled for the NEON
registers q4-q7 with the vpush instruction, except that they needed
to be preserved on all platforms, not only on one particular platform.

This allows removing the XMMREG_PROTECT_* hacks, which can
easily fail if the compiler chooses to use the callee saved
xmm registers in an unexpected spot.
Remove XMMREG_PROTECT

This isn't necessary any longer, when all the assembly routines
take care of restoring registers as necessary.
Add a test of encoding multiple spatial layers

This increases the coverage of the unit tests.
add IdctResAddPred unit test for decoder
Merge pull request #526 from licaiguo/IdctResAddPred-pr

add IdctResAddPred unit test for decoder
Merge pull request #519 from mstorsjo/push-xmm-registers

Backup/restore the xmm6-xmm15 SSE registers within asm functions on win64

Reviewed by zhiliang
Fix the comment in welsEncoderExt.h about the EncodeFrame return value

This was changed in 36d56b6638 in the public api, but the
internal implementation header was missed and left inconsistent.
Merge pull request #528 from mstorsjo/fix-comment

Fix the comment in welsEncoderExt.h about the EncodeFrame return value
Merge pull request #511 from mstorsjo/remove-unused-define

Remove the unused FORMAT_COFF define
update master to latest and add ut of intraprediction in decoder
Merge pull request #530 from licaiguo/DecoderIntraPredUT

update master to latest and add ut of intraprediction in decoder
modify WELS_CLIP1 to be inline functions
Add missing includes of time.h

This fixes building on MSVC.
remove unncessary cast
Remove an unused gitignore entry

Nothing within this project will generate .ko files.
Merge pull request #531 from mstorsjo/add-missing-includes

Add missing includes of time.h
    fix bug that there is no output in encoder console
Merge pull request #534 from ruil2/encoder_ret

    fix bug that there is no output in encoder console
Merge pull request #533 from mstorsjo/remove-unused-gitignore-entry

Remove an unused gitignore entry