1. Split out the download function from the mmap'ing of the plane(s)
2. Make the code mmap() all the planes
3. Determine size using lseek() as documented by libdrm, instead of
trying to guess it from the format.
4. Fix some bugs, like switcheroos of width and height
Tries to sanitize the dmabuf to conform to the values expected
by Vulkan/EGL which should also be the values expected by
Wayland compositors
We put these sanitized values into the GdkDmabufTexture, by
sanitizing the input from GdkDmabufTextureBuilder, which are
controlled by the callers.
Things we do here:
1. Disallow any dmabuf format that we do not know.
1. Treat the INVALID modifier the same as LINEAR.
2. Ignore all other modifiers.
3. Try and fix various inconsistencies between V4L and Mesa,
like NV12.
*** WARNING ***
This function is not absolutely perfect, you do not have a
perfect dmabuf afterwards.
In particular, it doesn't check sizes.
GdkDmabuf is a struct encapsulating all the values of a dmabuf, so
nothing to see here.
GdkDmabufDownloader is a vtable for a thing that can download dmabufs.
For now only one implementation exists, so this just looks like a ton
of work for no benefit.
The only neat thing is that gdkdmabuftexture.c got a whole lot tidier.
