Clear the texture used by PP shaders prior to use.
We were generating a texture without ever setting the data to a known value.
This happened on the old code as well, just that PP shaders are receiving some love and people are using it and noticing some of its issues.
This takes advantage of a new GL 4.4 feature for clearing textures if it is available. So if someone is resizing their window retardly alot, it should
be fairly quick at clearing the texture.