Kudos for taking also the texture colorspace issue into account - I've found people new to real time programming tend to find this initiaĺly one of the most bysantine minutiae one absolutely needs to get right
If there are issues you could report them on https://github.com/tomaka/glium/issues
https://en.wikipedia.org/wiki/SRGB
However, one cannot do computations on an image in sRGB colorspace since the colorspace is not linear - the values will turn out wrong. I.e. imaging algorithms expect source data to be in linear colorspace, and if fed sRGB data the result will turn out more or less wrong.
Thus, when doing any weighting calculations etc. on a set of samples the samples must be in linear (e.g. RGB) colorspace.
Your OpenGL implementation may or may not handle this automatically for you if you specify the image to be in one of the sRGB texture formats. https://www.opengl.org/registry/specs/EXT/texture_sRGB.txt
So, in the general sense, if doing any handcoded imaging routines, and reading e.g. PNG image, one needs to convert it to linear colorspace before processing, and back to sRGB before writing to disk/displaying.
'The importance of being linear' is the best introduction to the topic I know: http://http.developer.nvidia.com/GPUGems3/gpugems3_ch24.html