At the moment almost all device packages force installation of the Mesa
drivers, even when they are not used by any application (for example on
a minimal headless installation with "none" or "console" UI).
Omitting mesa-dri-gallium from such installations saves about ~150 MiB
of disk space (469 MiB -> 317 MiB rootfs for minimal installation on
arrow-db410c).
The "classic" drivers have been removed from Mesa so only one mesa-dri-
package exists now: mesa-dri-gallium contains all Mesa drivers,
llvmpipe, freedreno, lima, panfrost, Intel (iris/crocus), ...
This means we can easily create an install_if package in
postmarketos-base that installs that driver package only if needed
(= only if another package requires the "mesa" package).
Strictly speaking the install_if could be restricted further since
mesa-dri-gallium is only needed by "mesa-egl", "mesa-gbm" and "mesa-gl"
but not e.g. the Vulkan drivers. Having three postmarketos-base
subpackages (one install_if for each of them) seems a bit
overengineered, though. "mesa" is a common dependency of all three
of them, so using install_if="... mesa" should be good enough.
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