Support for multiple passes

[FelixSelter]

I would like to get a feature where we can specify multiple .frag files like glslViewer 1.frag 2.frag 3.frag.
glslViewer could then connect them by storing the output of 1 in a texture and pass this as input to 2 and pass the output of 2 to 3.

[wraybowling]

This is already a feature. https://github.com/patriciogonzalezvivo/glslViewer/wiki/GlslViewer-DEFINES#buffers-and-render-passes