* added wip scissor testing implementation (state+rectangle setting in pipeline and framebuffer clearing already work, rendering pipeline implementation still wip)
 * the "active" pipeline can now be set in the oclraster class (this is necessary since misc pipeline external classes must access certain pipeline states, e.g. framebuffer must be able to retrieve the scissor test state and rectangle; furthermore, this decreases the function call count in the main rendering loop by one -> pipeline swap doesn't have to (and shouldn't) be called directly any longer); -> updated all samples to make use of this
 * added the possibility to set the default/initial kernel_spec when constructing a oclraster_program
 * fixed orthograpic rendering when a different framebuffer with a different size gets bound while orthographic rendering is active (the camera state must be recomputed due to the different framebuffer size)
 * fixed rendering of very small 2d primitives (< 1px)
 * removed unnecessary/unused opengl functionality
 * set depth testing to "less or equal" for now
 * oclraster_support:
   * use the oclraster scissor testing everywhere
   * fixed gui main framebuffer -> default framebuffer blending/blitting (correct size + actually set the vertex and index buffer)
   * fixed rounded rectangle rendering
   * fixed depth of rendered 2d primitives
   * misc cleanup