pixel-array: Only allow allocation of arrays with a 2D size

pixel-array: Only allow allocation of arrays with a 2D size

This removes cogl_pixel_array_new which just took a size in bytes.

Without the image size and pixel format then the driver often doesn't
have enough information to allocate optimal GPU memory that can be
textured from directly. This is because GPUs often have ways to
spatially alter the layout of a texture to improve cache access patterns
which may require special alignment and padding dependant in the images
width, height and bpp.

Although currently we are limited by OpenGL because it doesn't let us
pass on the width and height when allocating a PBO, the hope is that we
can define a better extension at some point.