dummy: make limits.max_group_threads sane

This was UINT32_MAX, which explodes some of the shader logic. Pick
something more reasonable.

src/dummy.c

@@ -40,8 +40,8 @@ const struct pl_gpu_dummy_params pl_gpu_dummy_default_params = {
         .min_gather_offset  = INT16_MIN,
         .max_gather_offset  = INT16_MAX,
         .max_shmem_size     = SIZE_MAX,
-        .max_group_threads  = UINT32_MAX,
-        .max_group_size     = { UINT32_MAX, UINT32_MAX, UINT32_MAX },
+        .max_group_threads  = 1024,
+        .max_group_size     = { 1024, 1024, 1024 },
         .max_dispatch       = { UINT32_MAX, UINT32_MAX, UINT32_MAX },
         .align_tex_xfer_stride = 1,
         .align_tex_xfer_offset = 1,