Zink: Fix clear being missed when using emulated draws in zink_blit

zink_kopper_acquire_readback will flush any outstanding clears, this means that the current clears need to be applied first before calling zink_kopper_acquire_readback. This was already done for native_blit and native_resolve, also do this for the emulated draw path. Seen as intermittent failures in cts case GTF-GL33.gtf21.GL2FixedTests.buffer_clear.buffer_clear.