gallium/meson: Deconflate swrast/softpipe/llvmpipe

This adds explicit names for softpipe and llvmpipe to the gallium driver
list. "swrast" is treated as a compatibility name that selects both. We
clarify how lavapipe depends on (just) llvmpipe, and we make it possible
to build llvmpipe without softpipe.

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/27607>

meson_options.txt

@@ -89,10 +89,11 @@ option(
     'auto', 'kmsro', 'radeonsi', 'r300', 'r600', 'nouveau', 'freedreno',
     'swrast', 'v3d', 'vc4', 'etnaviv', 'tegra', 'i915', 'svga', 'virgl',
     'panfrost', 'iris', 'lima', 'zink', 'd3d12', 'asahi', 'crocus', 'all',
+    'softpipe', 'llvmpipe',
   ],
   description : 'List of gallium drivers to build. If this is set to auto ' +
                 'all drivers applicable to the target OS/architecture ' +
-                'will be built'
+                'will be built.'
 )
 
 option(