Memory limits due to texture memory

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Nvidia cards don't allow textures bigger than 512MB. Because this code uses 
texture memory, this imposes a limit on the sizes of various buffers. For 
example if your layer has too many filters (such that its output size exceeds 
512MB), the code will crash.

TODO: add non-texture-using routines to bypass this. 

Original issue reported on code.google.com by akrizhev...@gmail.com on 25 Jul 2014 at 1:28