Fix initializing buffer with unaligned data

In wgpu 0.7, this was a valid operation, the zero padding was done by hand. 
However in gfx-rs#872 this was removed as an optimization since buffers are zero padded, however the slice end was taken from buffer.slice which has to be aligned when mapped.

src/util/device.rs

@@ -65,9 +65,7 @@ impl DeviceExt for crate::Device {
 
             let buffer = self.create_buffer(&wgt_descriptor);
 
-            buffer
-                .slice(..unpadded_size)
-                .get_mapped_range_mut()
+            buffer.slice(..).get_mapped_range_mut()[..unpadded_size as usize]
                 .copy_from_slice(descriptor.contents);
             buffer.unmap();