gpu/utils: allow non-texel emulated format slices

OpenGL has emulated texture formats that don't need texel storage buffer
emulation, so support this case also.

Note that this change does not actually do anything yet, but I plan on
re-using this helper in the (near) future.

src/gpu/utils.c

@@ -473,8 +473,7 @@ int pl_tex_transfer_slices(pl_gpu gpu, pl_fmt texel_fmt,
     size_t max_size = params->buf ? gpu->limits.max_buf_size : SIZE_MAX;
 
     pl_fmt fmt = params->tex->params.format;
-    if (fmt->emulated) {
-        pl_assert(texel_fmt);
+    if (fmt->emulated && texel_fmt) {
         size_t max_texel = gpu->limits.max_buffer_texels * texel_fmt->texel_size;
         max_size = PL_MIN(gpu->limits.max_ssbo_size, max_texel);
     }