Add errors for VS.

- 4716 function must return a value.

- 4700 uninitialized local variable used.
Add gradient test to intra sad.
Add a benchmark for strategies.

- usage: kvazaar_tests -v -s speed

Conflicts:
	build/kvazaar_tests/kvazaar_tests.vcxproj
	build/kvazaar_tests/kvazaar_tests.vcxproj.filters
	tests/tests_main.c
Move test strategy initialization to its own module.
Merge branch 'speed-test'