Added support for Multiple Render Targets, to Dali::FrameBuffer.

Added support for Multiple Render Targets, to Dali::FrameBuffer.

- A maximum of 8 colour attachment are currently supported (as the minimum
  required by the OpenGL standard);
- Repeated calls to AttachColorTexture() adds subsequent color attachments;
- Added a GetColorAttachment() overload that takes an index; an empty handle
  is returned when an invalid index was provided;
- An error is logged if the maximum is exceeded;

Change-Id: I0b4cc593ab5e40b17dc427956cd78950ab49e5ae
Signed-off-by: György Straub <g.straub@partner.samsung.com>