vrend: Support R16G16B16X16_FLOAT
Running vkcube-wayland with Weston makes use of the R16G16B16X16_FLOAT format, so support it.
Extending and replacing !1375 (closed) by adding the conversion from RGBA data that is send by the mesa guest to RGB data that is used as internal format by the host GL/GBM.
Edited by Gert Wollny