VideoCommon: rename PixelShaderManager to ConstantManager

This is the first step of the merge with VertexShaderManager
VideoCommon: move uniform declaration to ConstantManager.h
VideoCommon: merge VertexShaderManager into ConstantManager
d3d: only use pixel shader constants as both are identical
VideoCommon: drop depthparam as zbias[1] is the same

depthparam was only accessable in vertex shader, zbias only in pixel, so
we're used to upload it twice. As both shader now access the same content,
we don't need to upload it twice any more.
VideoCommon: don't define uniform struct more than once

This will merge the uniform struct definition from ConstantManager and ShaderGenerator.