iRectangle2D: area() is not constexpr, because iPoint2D::area() isn't too.
iRectangle2D: area() is not constexpr, because iPoint2D::area() isn't too.

Fixes gcc build.

LebedevRI/rawspeed,LebedevRI/rawspeed,darktable-org/rawspeed,darktable-org/rawspeed,LebedevRI/rawspeed,darktable-org/rawspeed,LebedevRI/rawspeed,LebedevRI/rawspeed,darktable-org/rawspeed,darktable-org/rawspeed