cmake: Fix FMA4 detection
Fix 256-bit support which was broken in commit 8e092640e8
math_int: Rename adds and subs to add_sat and sub_sat respectively
README: Remove the roadmap section as the wiki covers that much better
doc: Document the non-empty expression types that functions may return
null: Rename convert_mask() to unmask_mask() for consistency
test: Fix bitmask generation in blend tests
test: Use correct test helpers

TEST_ALL_COMB_HELPER1 and TEST_ALL_COMB_HELPER1_T were implemented as
corresponding 2-element macros. This commit fixes the 1 element helpers
and switches all users to the 2-element ones.