glsl: fall back to inexact function-match

glsl: fall back to inexact function-match

In GLES, we currently either need an exact match with a local function,
or an exact match with a builtin.

However, if we add support for implicit conversions for GLES shaders,
we also need to fall back to a non-exact match in the case where there
were no builtin match either.

Luckily, we already have a variable ready with this, so let's just
return it if the builtin-search failed.

Signed-off-by: Erik Faye-Lund <erik.faye-lund@collabora.com>
Reviewed-by: Tapani Pälli <tapani.palli@intel.com>