Add Image read/write support

[cbaggers]

Its part of the GL api so gotta do it some day

[azimut]

one of the uses for this is voxel based global illumination by writing to a 3d texture (https://github.com/Friduric/voxel-cone-tracing/tree/master/Shaders/Voxelization) i monkey patched things together so I leave some notes for a proper implementation:

• varjo needs a type for it
• varjo needs to write a layout for each image-3d like

layout(RGBA8) uniform image3D texture3D;

• cepl needs to add a call to %gl:bind-image-texture additionally to what it does for samples on the defpipeline-g
• optional: %gl:clear-tex-image is needed for vxgi

[cbaggers]

Cool, thanks @azimut. Maybe I'll try do this after layered attachments.