DRI3 support with AMDGPU and other open source GPU drivers

[DRC]

The dev branch and 3.2 Evolving pre-release builds
(https://turbovnc.org/DeveloperInfo/PreReleases) now contain an
implementation of the DRI3 X extension, which can be enabled by passing
'-drinode {DRM-render-node}' (where {DRM-render-node} is, for example,
/dev/dri/renderD128) to /opt/TurboVNC/bin/vncserver. This only works
with Mesa-based GPU drivers, including AMDGPU. It does not work with
nVidia's proprietary drivers.

The main purpose of this feature is to allow AMDGPU users to use Vulkan
with GPU acceleration in TurboVNC. (Whereas nVidia's Vulkan
implementation does something VirtualGL-like in an X proxy environment,
AMD's implementation simply doesn't work without a DRI3 extension.) The
performance is reasonable for casual GPU users, but VirtualGL is faster
and has more features for OpenGL applications (particularly professional
applications), and VGL is still required for nVidia users. Our DRI3
implementation is based on the implementation in KasmVNC 1.2.0, with
minor changes (most of them cosmetic.)

Feedback is welcome and appreciated.