Add DDS saving

[radarhere]

Helps #4864 by adding DDS saving. https://docs.microsoft.com/en-us/windows/win32/direct3ddds/dds-header was used as a reference.

Also, a minor cleanup change. Uses this existing line

Pillow/src/PIL/DdsImagePlugin.py

Line 50 in 45003b7

DDPF_RGB = 0x40

to make

Pillow/src/PIL/DdsImagePlugin.py

Lines 130 to 131 in 45003b7

	if pfflags & 0x40:
	# DDPF_RGB - Texture contains uncompressed RGB data

clearer.

if pfflags & DDPF_RGB:
    # Texture contains uncompressed RGB data

[radarhere]

I've pushed another commit for RGBA saving.

While you would look at the spec and conclude that you should be able to specify the order of the channels arbitrarily using the bit masks, checking the results with ImageMagick I didn't find this to work. Instead, I've swapped the RGBA channels before saving so that it becomes ARGB, the example for the bit masks - https://docs.microsoft.com/en-us/windows/win32/direct3ddds/dds-pixelformat.