This patch ensures that we never request a zero-sized virtual
framebuffer from nitpicker even when instantiating the object with zero
width or height. It therebu removes the burden of handling the resulting
invalid framebuffer dataspace from the user of the Nitpicker_buffer
utility.
The headers 'texture_rgb565.h' and 'texture_rgb888' contain
template specializations needed for using the 'Texture::rgba' function
for the respective pixel formats. The specializations were formerly
contained in application-local code.