Clear background alpha for gradient/textured backgrounds.

When vtkRenderer's background is set to a single solid color, we render the frame buffer has alpha set to 0 for the background. However, if textured background or gradient background was set, the frame buffer would have the alpha set to 1.0 (or whatever the texture said it was). This inconsistency makes it hard for applications like ParaView to grab the frame buffer and use the alpha value as an indicator for background consistently. This fixes that issue. We clear the alpha buffer after rendering gradient or textured background.