Retire the GL_GENERATE_MIPMAP hack for FlatInput.

Retire the GL_GENERATE_MIPMAP hack for FlatInput.

I found very similar workaround code for this bug in Chromium,
with the following comment:

  // Workaround for Mac driver bug. In the large scheme of things setting
  // glTexParamter twice for glGenerateMipmap is probably not a lage performance
  // hit so there's probably no need to make this conditional. The bug appears
  // to be that if the filtering mode is set to something that doesn't require
  // mipmaps for rendering, or is never set to something other than the default,
  // then glGenerateMipmap misbehaves.

Going back all the way to the point in which this code was written,
it is indeed true; we called glGenerateMipmap(), and then right afterwards
set the mode to GL_LINEAR_MIPMAP_NEAREST. Since then, the code has been
reorganized and moved around a lot, and now we set the mode long before
the first call to glGenerateMipmap(), and thus we can retire the hack;
simply generate mipmaps on-demand, and that's the end of it. I tested
with the Mesa 8.0.x version where I originally saw this bug, and it passes
flat_input_test without any problems (well, actually all tests except
the tests for deconvolution sharpen, whose shaders are too big for it).

This is nice not only because it gives us a less hacky structure, but also
because GL_GENERATE_MIPMAPS is a nightmare for the driver to handle;
several edge conditions are tricky, from what I've been told.