This adds support in headless rendering for more
compact 24-bit RGB image storage in 3-byte pixels
instead of 4 bytes.

There is a conversion necessary to accomodate Cairo,
which requires 4-byte pixels, but that is relatively.

Early tests show no loss of performance at runtime.

Change-Id: I3919f7c56d14d09e67f163f035b4c7683b49087c