Upgrade to latest piston-graphics about glium_graphics

[gaborigloi]

I had a go at this (but not the sRGB conversion): https://github.com/gaborigloi/glium_graphics/tree/latest-graphics, this compiles with the latest piston2d-graphics.
To upgrade, most of the time I just followed what the opengl_graphics backend does.
I also updated piston-texture to the latest version.

I noticed two things:

• The definition of alpha blending in Piston is different from alpha blending in glium for the alpha channel.
  piston2d-graphics says new_dest_alpha = src_alpha + dest_alpha, but what glium does is
  new_dest_alpha = src_alpha * src_alpha + dest_alpha * (1 - src_alpha).
  Therefore I could not use glium::Blend::alpha_blending(), had to construct the Blend manually.

• For graphics::draw_state::Blend::Invert, I used this blending:

  new_dest_color = (ref_color - dst_color) * src_color,

  but the equation in the rustdoc of Invert is different, it says

  new_dest_color = ref_color - src_color.

Also, I'm not sure what formatting to use, I'm still new to Rust. Should I just run rustfmt to format the code, or is the current formatting ok?

[gaborigloi]

Opened a pull request: #112

[bvssvni]

I think piston-graphics used the alpha equation from Gfx, but after doing some research it seems neither Glium or Gfx use the correct one. Opened PistonDevelopers/graphics#1042

[bvssvni]

For now, we could use new_dest_alpha = src_alpha + dest_alpha.