Merge tag 'v6.6.3' into v6.6.x

Conflicts:
	NEWS.txt
Merge remote-tracking branch '_max/v6.6.x' into v6.6.x

Conflicts:
	NEWS.txt
SDL/TopCanvas: ignore -Wcast-align
Screen/Memory/Optimised: add MMX alpha blending

Speedup by a factor of 5.
Screen/Memory/MMX: use _mm_setzero_si64()
Screen/Memory/MMX: move code around
Screen/Memory/MMX: support BGRAPixelTraits
Screen/Memory/Optimised: remove unused typedefs
Memory/Canvas: merge code to DrawPolyline()
Screen/Memory/Bresenham: move branches outside loops

The swapdir variables don't change while in the loop, so don't keep checking them.
Memory/RasterCanvas: optimised polygon fill

Collaboration with Max Kellermann
RasterCanvas: restore order of points in ClipLine
Makefile: enable libpng and libjpeg for USE_FB=y

This was missing, but was never noticed, because this was hard-coded
for the Kobo.
FB/TopCanvas: "bpp" is bytes per pixel, not bits
FB/TopCanvas: support RGB565 output
Makefile: remove Kobo + SDL option
Makefile: remove obsolete variable SDL_PKG
Makefile: move rules from fb.mk to libpng/jpeg/freetype.mk
Circling diameter and full circle time infobox (enhancement #2939)

Adds a Cirle diameter infobox which displays estimated circle diameter and
full circle flight time. Useful for evaluating best thermalling mode with a
glider at different wing loading.
refactored turn_rate* from fixed to Angle