Fixing TransparentBuffer when AlphaBits is not set by the user

[MarioGK]

Summary of the PR

This fixes a bug when setting TransparentBuffer to true and leaving PreferredBitDepth null, the transparent buffer would not be applied correctly.

You can do this to "hotfix" the problem until this is merged: options.PreferredBitDepth = new Vector4D<int>(8);

[dnfadmin]

All CLA requirements met.

[Beyley]

Does this occur on the SDL backend too?

[Beyley]

Given the message here it seems this does not need to be fixed on SDL too, so this should be ready to merge