Fix shader sampler to texture unit binding

Also, setting a non existing texture is no longer an error, only a
warning.
In the future binding samplert to texture units should be revised. It
makes no sense to store those as part of the uniform state. Store them
in Program, also makes it easier to use some resister allocation like
algorithm to minimize texture changes.
Change printing of missing file message
Add default uniforms to DefaultImport.renderstate