fix check for flush_mapped_ranges in map_buffer

`flush_mapped_ranges` needs to be called when mappings are not coherent.
We can also omit flushing for write-mapped buffers since we always flush them on unmap.

wgpu-core/src/device/mod.rs

@@ -349,7 +349,7 @@ fn map_buffer<A: HalApi>(
             (uninitialized.start - offset) as usize..(uninitialized.end - offset) as usize;
         mapped[fill_range].fill(0);
 
-        if mapping.is_coherent {
+        if !mapping.is_coherent && kind == HostMap::Read {
             unsafe { raw.flush_mapped_ranges(raw_buffer, iter::once(uninitialized)) };
         }
     }