A small handful of minor changes to the sliced texture transfer code and related functions

Nothing substantial to see here, I was originally planning on rewriting the sliced texture code to also handle misaligned formats, but this turns out to be a lot trickier to get right than I thought, and I'd rather avoid breaking everything right now.

src/vulkan/gpu_tex.c

@@ -916,12 +916,11 @@ bool vk_tex_upload(pl_gpu gpu, const struct pl_tex_transfer_params *params)
         if (params->callback)
             vk_cmd_callback(cmd, tex_xfer_cb, params->callback, params->priv);
 
-        CMD_FINISH(&cmd);
+        bool ok = CMD_FINISH(&cmd);
 
         // Finally, dispatch the (texel) upload asynchronously. We can fire
         // the callback already at the completion of previous command because
         // these temporary buffers already hold persistent copies of the data
-        bool ok = true;
         for (int i = 0; i < num_slices; i++) {
             if (ok) {
                 slices[i].buf_offset = 0;
@@ -967,9 +966,10 @@ bool vk_tex_upload(pl_gpu gpu, const struct pl_tex_transfer_params *params)
         if (params->callback)
             vk_cmd_callback(cmd, tex_xfer_cb, params->callback, params->priv);
 
-        CMD_FINISH(&cmd);
+        return CMD_FINISH(&cmd);
     }
-    return true;
+
+    pl_unreachable();
 
 error:
     for (int i = 0; i < num_slices; i++)
@@ -1100,11 +1100,10 @@ bool vk_tex_download(pl_gpu gpu, const struct pl_tex_transfer_params *params)
         if (params->callback)
             vk_cmd_callback(cmd, tex_xfer_cb, params->callback, params->priv);
 
-
-        CMD_FINISH(&cmd);
+        return CMD_FINISH(&cmd);
     }
 
-    return true;
+    pl_unreachable();
 
 error:
     for (int i = 0; i < num_slices; i++)