Make shader binary alignment handling simpler and more robust

[RandomShaper]

• Readability of code is improved.
• Padding bytes containing in the shader binary file guaranteed to be zero (garbage data there is not the end of the world, but makes serialization non-deterministic and manual inspection of the data a bit less convenient).
• Prevention of read-past-EOF is harmonized across rendering drivers (both now use ERR_FAIL instead of CRASH_COND, which is better since you'd rather have a missing shader than a fully crashing game).

Bonus:
Also simplified the rounding to block size in image size calculations.

[akien-mga]

Thanks!