Support Mesa off-screen software rendering with minimal platform deps

[kmcallister]

Mesa supports a mode where OpenGL renders to an off-screen buffer in main memory. This is pure computation in userspace; there's no window system or OS dependencies. There are several ways we could use this in Servo:

• Headless ref tests without Xfvb
• Headless embedders that want to save raster output. (For the vector part of this see #3788.)
• Extreme portability. In this mode you can use Servo on any platform that has a framebuffer, e.g. EFI, Linux fbdev, game console homebrew
• High-security mode where all of Servo is heavily sandboxed, except a minimal UI task that communicates with the engine over a well-defined IPC protocol and receives bitmaps to display. At this point it really is a VM and an OS that you can VNC into :D

We can enable this in desktop Servo if we switch to glutin (#3579). But maybe we should only think of it as a feature for other embedders.

[kmcallister]

It would be pretty awesome to teach a systems course where you write an OS in Rust with a TCP/IP stack and the final project is to port Servo to it.

[SimonSapin]

Sounds great, requiring an X server is a common pain point when doing stuff on a headless server.

[glennw]

This happened.