Some drivers apparently need private constants, or don't have an efficient
immval packing. For example, MacOS seems to need 1 float for each different
relative addressing offset. fglrx has the same issue, although it is more
efficient in general

Previously this worked on most drivers because the 16 + 4 reserved int and
bool constants kept the problem hidden. Now that we are more aggressive with
uniforms we have to keep free room for some drivers.