minimum changes to fix BindImageTexture

[mp3butcher]

re-add Texture changes of
#466
and fix issue
http://forum.openscenegraph.org/viewtopic.php?t=17102
by making _modifiedcount Texture attribute (see commit log)

[openscenegraph]

Making _modifiedCount part of the base class is broken because you end up having to duplicate the data structure for the array version of texture.

Also I can grasp how 12 changed files could be at all a minimal change set. You've merged three set of changes into one commit.

Adding the texture unit as into BindImageTexture just feels like this class is getting more a more tightly coupled state that breaks the orthogonal design that the OSG and OpenGL try to maintain, the fact you are having to do this tight coupling suggests that approach is far from idea and may need revising at a fundamental level.

I'm closing this PR as it just contains too many different problems.

[mp3butcher]

Why do you close a pr without discussing it?
Try to respect my insights as i respect yours

As i mentionned in the commit the semantic of _modifiedCount is not clear:

• in unlayered Textures it refers to texture object dirtiness
• in layered Textures it's used as a hint not to upload unchanged image layers (there no per context per layer gl object..)

Make _modifiedcount a Texture attrib make clear to everyone that _modifiedCount represents "texture object modified count" and not a context-unrelated-inner-tricks-not-to-upload-image-already-there
Given this semantic there's no data duplication....

Concerning how I designed BindImageTexture, we already have that discussion it's not a textureatrribute imageunit is not textureunit!!
But in the case texture target is dirty we need a tu otherwise we hack the current active textureunit and it can lead to problems..
the solution of a "virtual-texture-attribute" seams the wisest