Fix image format layouts

This series fixes the format specifiers for images on GLES by either emitting a default format when none is given or by enabling NV_image_formats when requires. It also drops the write flag on GLES when the shader actually reads from the image, since on GLES this is interpreted as writeonly. For image types that can be written and read this is of no further consequence, and in the other cases it was forbidden to read from and write to the image from within the same shader anyway, so that we simply switch the logic.