Implement min/mag texture filtering

[kettek]

These changes allow users to call SetTextureMinFilter and SetTextureMagFilter on the MasterWindow to change how textures are
filtered for rendering. This allows pixel-perfect rendering via giu.TextureFilterNearest.

[AllenDang]

@kettek I think those two functions should be added to renderer interface only. Because they are kind of rarely be used, which could remain the simplicity of MainWindow structure.
So the usage could be like this

func main() {
  wnd := g.NewMasterWindow(...)
  g.Context.GetRenderer(). SetTextureMinFilter(...)
  g.Context.GetRenderer(). SetTextureMagFilter(...)
  ...
}

Do you think this is doable?

[kettek]

That would certainly be simpler and would be functionally equivalent -- I'll make adjustments.

[kettek]

Now adjusted to remove MasterWindow methods.

[AllenDang]

@kettek Thanks! Can you add a demo about how to use those two functions?

[kettek]

I've created an example in examples/texturefiltering. One problem I noticed is that sometimes the program will freeze. I think this is due to a race condition stemming from the goroutine for creating the image textures -- giu is accessing the texture variables before they are guaranteed to be populated with the actual texture data. Unfortunately using channels didn't solve the problem (it just froze it entirely... which is strange).

I left commented out in the code additional buttons for setting some other mipmap modes -- I have disabled them since their use causes non-image imgui draw calls to not be rendered.

[kettek]

To note, I think this freeze is related to #30

[AllenDang]

I tried to solve this once, and I did fix it with giu, but unfortunately the upstream lib GLFW also has same problem. I need to create a PR to GLFW when I got time.