DropoutLayer: Use nonsymbolic shape for dropout mask if possible

This fixes the second point of #64. When the dropout layer's input shape is fully specified (read: the output shape of the dropout layer's input layer), it uses that for the dropout mask instead of the symbolic shape of the input tensor.
(No tests are added because it doesn't affect results, it's just potentially faster or less memory-hungry. Disclaimer: I didn't even run the existing tests because I currently cannot connect to a suitable machine; I'll let Travis figure out if everything is fine.)