Do not corrupt textures between FBOs

Previously, if we set a texture (e.g. tex0) and some parts of it
were transparent, all the transparent things would have been changed
when doing fxobject[0].draw(...). The transparent parts would have
had their alpha channel mixed with the black color due to the
ofClear(0,0) call in setTexture. Now we no longer destroy textures.
Merge pull request #20 from OliverUv/ofxfxobject-improvement

ofxFXObject improvement
Merge pull request #18 from OliverUv/fix-transparency

Do not corrupt textures between FBOs
Fixing credits
gif thumbnail
Enable GL-version specific shader source loading

- Load now uses ofShader to load shaders from path,
  so we know for sure it will work the same way.
- Ensure FBOs are set to correct width/height when
  allocate is called.
- Can set shader source for GL ES/2/3 by setting
  specific strings in constructor, or by calling
  loadVersioned.
- Can reload shaders from file.
ofxBlur uses versioned shader strings
Add test projects
ofxBloom uses versioned shader strings
ofxPassthrough (test) uses versnd shader strings
Remove environment files.