DescriptorPool::computeTotalFreeSize() was adding the size
of the last element to its available memory size, rather
than subtracting it, which was causing the DescriptorPool
to return the wrong error (VK_ERROR_FRAGMENTED_POOL
instead of VK_ERROR_OUT_OF_POOL_MEMORY).

Bug: b/167617619
Change-Id: Ie60c2fe521b0d233b32504b446af1715361f5c21
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/47549Reviewed-by: Nicolas Capens <nicolascapens@google.com>
Kokoro-Result: kokoro <noreply+kokoro@google.com>
Tested-by: Alexis Hétu <sugoi@google.com>