wined3d: Clear the constant list before finding constants.

wined3d: Clear the constant list before finding constants.

shader_get_registers_used is delayed until compile time for some 1.x
shaders, mostly to wait for the right vertex declaration to be
set. This means that on a recompile it will be run again, adding
another instance of each local constant, which in turn causes compile
errors because of constant redeclaration. Just purging the lists
before finding the constants is a simple and reliable solution.