This is a C++11 feature that allows the compiler to check that a virtual
member declaration overrides a base-class member with the same signature.
Also src/blitter/32bpp_anim_sse4.hpp +38 is no longer erroneously marked
as virtual despite being a template.
Create a new blitter mode: 32bpp-sse2-anim, which is 32bpp-anim + this.
32bpp-sse2-anim is now used by default where 32bpp-anim would have been.
Also use this with the 32bpp-sse4-anim blitter.
See issue #6469.
Henry Wilson