test whether image fits into CUDA textures

[griwodz]

Description

Several recent bug reports are concerned with images that exceed the CUDA Texture size, but where PopSift just says "out of memory".

This PR adds a method testTextureFit() to class PopSift. This test can be called explicitly to test whether PopSift can be used for a given image and downscale factor, but even if it is not called explicitly, it is called during enqueue(). If an exceeded texture size is detected during enqueue(), NULL will be returned and a multi-line error string is printed on cerr to explain the error (because there are 2 different limits: (linear texture for image loading and layered texture for DoG pyramid creation).

The PR adds also the method testTextureFitErrorString() to get the multi-line explanation as a string for the caller's own purposes.

Features list

New method PopSift::testTextureFit.
New method PopSift::testTextureFitErrorString.

Provides explanations and hints for errors #77 and #88.

Implementation remarks

PopSift::enqueue changes behaviour. It can now return NULL if it will be impossible to process the given image size.

[griwodz]

Sorry about the force push, I added [popsift] to the commit string.

[simogasp]

LGTM
I just fixed very minor syntax stuff (easier to make than writing on a review...)

[nguyengiang41]

Hello @griwodz ,
I test your PR in PopSIFT release 1.0 version. It seem that this test is just taken in account the limit of graphic card to process image, it does not taken in account the VRAM available , which could cause the same error message "out of memory" when using cudaMalloc3DArray.
Could you confirm ?