mesa: add an environment variable to default enable INTEL_blackhole

Signed-off-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/7640>

src/mesa/main/context.c

@@ -885,6 +885,7 @@ init_attrib_groups(struct gl_context *ctx)
    ctx->ErrorValue = GL_NO_ERROR;
    ctx->ShareGroupReset = false;
    ctx->varying_vp_inputs = VERT_BIT_ALL;
+   ctx->IntelBlackholeRender = env_var_as_boolean("INTEL_BLACKHOLE_DEFAULT", false);
 
    return GL_TRUE;
 }