Changes

Reduce visibility -> package private

WGL: Add GetLastError in case external context/drawable can't be retrieved

Add GL_VENDOR to JoglVersion GL output

GLCanvas: Fix disableBackgroundErase/X11, make drawable creation more robust, doc Java2D/AWT properties.

On X11 disableBackgroundErase() must happen before native peer creation,
this patch issues it before and after super.addNotify().

Make drawable creation more robust, ie only create a drawable in case the size is > 0x0
and do this check/create at display/paint in case size it not yet determined.

Add documentation about Java2D/AWT properties impact on GLCanvas.

Make JUnit tests on AWT/GLProfile's more robust, while adding frame validate()
ensuring 1st paint will have a size, hence will create the drawable.
This is necessary for eg. AMD GPU's and GL context > 3.

Javadoc developer level must be 1.5 due to gluegen compile time plugin

NEWT WindowImpl: Null Check on childWindows (already destroyed)

NewtCanvasAWT: disableBackgroundErase() before (X11) and after (Windows) addNotify().

This change is in sync with 86c164950b0a0d351fc8af3884187b10201b6237

minor cleanup

JOGL: Proper handling of Read Drawable Support (if not supported), add query.

Read Drawable feature reflects the make context current API
having a seperate read drawable next to the write drawable (default).
  glXMakeContextCurrent(write, read, ..)

On X11 a ready drawable is only supported for GLX >= 1.3,
on Windows only if extension WGL_ARB_make_current_read is available,
on EGL it's always supported, on OSX not at all.

API cleanup GLContext:
  changes: setGLDrawableRead(GLDrawable) ->  setGLReadDrawable(GLDrawable)
  new:     isGLReadDrawableAvailable()
  new:     getGLExtensionsString();

Access qualifier cleanup.

GLContextImpl: GLVersionNumber moved out.