Adds CLI options for finer control of input resampling

First off, thank you for this lovely software!

This patch adds four command-line options to the encoder tool for increased control of input preprocessing resampling:

-resample_filter X Set resample filter kernel, default is box, filters: box, tent, bell, blackman, catmullrom, mitchell, etc.
-resample_filter_scale X Set resample filter kernel's scale, lower=sharper, higher=more blurry, default is 1.0
-resample_ifgt Only resample if the image is larger than width or height provided with -resample
-resample_aspect Keep aspect ratio of image while resampling. "fit" the image inside the -resample rectangle

I've tried my best to match the style of the existing code and have of course tested all this with a bunch of different inputs.

Examples

Resample inputs using lanczos3 filter with a 0.8 scale factor:

basisu -resample 512 512 -resample_filter lanczos3 -resample_filter_scale 0.8 a.png

Resample inputs to fit within 512x512, maintaining their aspect ratio:

basisu -resample 512 512 -resample_aspect a.png

Resample inputs only if the width is larger than 1024 or if the height is larger than 512:

basisu -resample 1024 512 -resample_ifgt a.png