nvk: look for gaps in descriptor pool
nvk: look for gaps in descriptor pool
fall back to looking for free gaps if we have run out of space in the descriptor pool.
Games like Transport Fever 2 allocates and frees from the same pool very often. This led to the game crashing with due to VK_ERROR_OUT_OF_POOL_MEMORY when we could not simply add it to the end.
This takes the approach that radv is also using to look for gaps when needed.