Xpra: Ticket #1552: nvenc SDK v8

The existing "nvenc7" codec compiles against it just fine. TODO:

rename the codec to just "nvenc" and use versioned checks with pkg-config.
check for new useful API extensions we can use
try to fix ticket:1260#comment:16 (especially now that the problem also occurs on other cards: ticket:1550#comment:4)
update the buildbot

Interesting things found in the SDK 8 headers:

encode from an opengl texture (NV_ENC_INPUT_RESOURCE_TYPE_OPENGL_TEX / NV_ENC_DEVICE_TYPE_OPENGL): could save us a lot of processing if we can stay on the GPU (#365)
NV_ENC_MEMORY_HEAP should not be used any more (maybe the alternative will fix the lockups?)
better "Long Term Reference" frames: could be used to send the whole window (including widgets, etc) encoded with lossless quality?
sps / pps changes: meh
"Motion Estimation" structures: meh
NV_ENC_PIC_PARAMS_HEVC can now be used

So not a lot there. r16083 renames the codec to "nvenc" and uses versionned pkgconfig checks.

This will do for now, the API fix will be tracked in #1550.

r16444 fixes RGB upload mode, so r16446 enables it by default again.

with NVFBC (#1317), the pixel buffers were bigger than the pixel contents, so r16456 sliced them to the right size for the device upload - but r16457 makes this redundant: the right place to fix this was in the CUDA image wrapper since the buffers are used elsewhere and we can't just go around and fix them all - this change is kept in place because the slicing should be (almost) free with numpy arrays
r16455: disable RGB mode on win32, no idea why it doesn't work "an API argument is invalid" error...