pixbuf_new_from_data or cairo take un-premultiplied rgb data

[Antoine Martin]

r4151 does this, but it's slow and ugly.

Do this in-place properly, or at least return a string we can use directly.

[Antoine Martin]

remove unnecessary conversions with mmap codepath only

[Antoine Martin]

The problem is that:

• we can get a string as data (from all encoders that can output with alpha: rgb, png, webp) or a class 'xpra.net.mmap_pipe.c_char_Array_409600' with mmap. A string cannot be used as writeable buffer but the c_char_Array can.
• the data we pass to pixbuf_new_from_data() must be string or read-only buffer, not bytearray

The patch above skips the conversions for the mmap codepath, but we need something that also addresses the double-copy from the standard codepath.

[Antoine Martin]

Done in r4843 (see changeset for details)

• mmap: zero copy
• string buffers: one copy with numpy (cannot be avoided)
• without numpy installed: two copies as before

More buffer work in #465