Thread/Handle: remove global namespace references

This breaks if one of these functions is a macro.
SDL/{Queue,Key}: downgrade SDL version checks from 2.0 to 1.3
po: update French translation
Util/TriState: new enum type
AirspacePolygon: lazy convex check

Convert AbstractAirspace::m_is_convex to a TriState and run the
GrahamScan on the first call to GetClearance().  Nobody else was
interested in the "m_is_convex" call, and GetClearance() has to do the
GrahamScan again anyway.
SearchPointVector: remove unused method IsConvex()
release v6.7.4
Merge tag 'v6.7.4'
RasterRenderer: Removed unused FAST_RSQRT patch

This code has been unused for 3 years now
RasterRenderer: Fixed Format
Screen/Ramp: Simplified bounds checks in ColorRampLookup() function
Screen/Ramp: Reduced array lookups
Screen/Ramp: Simplified ColorRampLookup()

One less indentation level for the complicated code
Screen/Ramp: Use pointer-based loop
Screen/Ramp: Extracted Interpolate() function
Screen/Ramp: Extracted ColorRamp::ToRGB8Color() method
Screen/Ramp: Added/Updated documentation
RasterRenderer: Added Documentation to MIX() function
RasterRenderer: Added Documentation to TerrainShading() function
RasterRenderer: Renamed Gmid/Gneighbor variables to center/neighbor