mesa: enable GBM; narrow gallium drivers to softpipe,llvmpipe,virgl

Enable GBM (required by the EGL/GBM/GLES2 surface used by Qt6/KWin).
Drop crocus/iris from the gallium driver set for the 26.1.4 build —
the Intel hardware drivers require DRM uapi surfaces that are not
available in the Redox sysroot yet. They MUST be restored when the
Intel DRM/redox-drm path matures (tracked in
local/docs/DRM-MODERNIZATION-EXECUTION-PLAN.md Stage 5); this is a
build-surface constraint, not a feature removal.

Also widen the -Wno-error set for the 26.1.4 cross-compile
(missing-prototypes, return-type, empty-body, incompatible-pointer-types,
int-conversion, format) — upstream Mesa enables -Werror by default and
the Redox sysroot headers trip these classes; the underlying warnings
remain visible in the build log.

local/recipes/libs/mesa/recipe.toml

@@ -75,15 +75,15 @@ fi
 cookbook_meson \
     -Ddri-drivers-path=/usr/lib/dri \
     -Degl=enabled \
-    -Dgbm=disabled \
+    -Dgbm=enabled \
     -Dglx=disabled \
-    -Dgallium-drivers=softpipe,llvmpipe,virgl,crocus,iris \
+    -Dgallium-drivers=softpipe,llvmpipe,virgl \
     -Dllvm=enabled \
     -Dplatforms=wayland \
     -Degl-native-platform=surfaceless \
     -Dshader-cache=disabled \
-    -Dc_args="['-Wno-error=implicit-function-declaration','-Wno-error','-std=gnu11','-Dstatic_assert=_Static_assert']" \
-    -Dcpp_args="['-Wno-error=implicit-function-declaration','-Wno-error']" \
+    -Dc_args="['-Wno-error=implicit-function-declaration','-Wno-error=missing-prototypes','-Wno-error=return-type','-Wno-error=empty-body','-Wno-error=incompatible-pointer-types','-Wno-error=int-conversion','-Wno-error=format','-Wno-error','-std=gnu11','-Dstatic_assert=_Static_assert']" \
+    -Dcpp_args="['-Wno-error=implicit-function-declaration','-Wno-error=missing-prototypes','-Wno-error=return-type','-Wno-error=empty-body','-Wno-error=incompatible-pointer-types','-Wno-error=int-conversion','-Wno-error=format','-Wno-error']" \
     -Dvulkan-drivers=swrast \
     -Dshared-glapi=enabled