Remove the clear allocation flag as it has little impact since there should
be very few allocation per redraw.
Make BLI_memblock_alloc and BLI_memblock_iterstep much more cache efficient
removing them almost entirely from performance profiles.
This is really close to BLI_mempool but uses an array to keep track of the
chunks of memory. There is no tagging necessary to clear the whole
structure so reuse is fast.
Naturally supports iteration but does not support freeing.
