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>

docs/envvars.rst

@@ -351,6 +351,11 @@ i945/i965 driver environment variables (non-Gallium)
    The success of assembly override would be signified by "Successfully
    overrode shader with sha1 <sha1>" in stderr replacing the original
    assembly.
+``INTEL_BLACKHOLE_DEFAULT``
+   if set to 1, true or yes, then the OpenGL implementation will
+   default ``GL_BLACKHOLE_RENDER_INTEL`` to true, thus disabling any
+   rendering.
+
 
 Radeon driver environment variables (radeon, r200, and r300g)
 -------------------------------------------------------------

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;
 }