Fill buffer fix

Fill buffer fix

memset was erroneously being used to copy 4 bytes at a time,
but the integer input value in memset is used as an 8 bit value,
so the fill was failing if the 4 bytes were not identical.

Tests: dEQP-VK.api.fill_and_update_buffer.*
Tests: dEQP-VK.memory.pipeline_barrier.host_read_transfer_dst.*

Bug b/118383648

Change-Id: I29cb5915ebd3773b4afbd89850c47a042fff6952
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/27872
Reviewed-by: Nicolas Capens <nicolascapens@google.com>
Tested-by: Alexis Hétu <sugoi@google.com>
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>