* redesigned/improved framebuffer clearing:
   * instead of using one generic (ulong4) clear color, use a separate clear color for integer and float image formats (this way, both can make full use of their precision/value range)
   * instead of converting the clear color down to the different image format types (for each pixel ...), already convert them on the host side and pass them in as kernel arguments
   * for simplicity there is still a generic framebuffer::set_clear_color function (which takes a double4) that will set both the integer clear color (via cast) and float clear color
 * made IMAGE_TYPE and IMAGE_CHANNEL enums usable as hashes
 * ensure that vector4 types that are used for images have the correct size
 * fixed README (the "./premake.sh pocl" part should have been in the FreeBSD section, not the Windows section)