- Mesa-devel: no longer require libOSMesa-devel, since it's now
built in Mesa-drivers; packages should require it directly now,
preferrably via pkconfig(osmesa) ...

- Switch dummy driver for mesa package from "auto" xorg driver (i965) to
gallium swrast driver

- update to 21.0.2
* many additions especially on the Radeon Vulkan (RADV) driver
front where sparse memory support is in place, AMD Smart Access
Memory / Resizable BAR optimizations, various RDNA 2
improvements, rapid packed math for ACO, and more.
Elsewhere in Mesa 21.0 there are continued RadeonSI
optimizations, many Intel ANV and Iris improvements, OpenGL 3.3
for Freedreno, DXGI Winsys was added and initial Direct3D 12
code for WSL, OpenGL 4.1 for Zink, and more.
- moved osmesa build to Mesa-drivers since swrast driver has been
removed from Mesa