If we have start = 4, count = 3, the highest dirty constant is the one with index 6. start + count gives 7,
so it already includes the zero-based array correction. Don't add an additional 1.

Additionally to the inefficiency of looking at one extra constant, this causes problems if the driver
rejects loading 257 constants on the initial load. In this case no constant is loaded if
GL_EXT_gpu_program_parameters is used.