Fix dynamic slice adjustment for frames with extreme aspect ratios

[sprangerik]

When rate control is disabled (RC_OFF_MODE), dynamic slice adjustment
in DynamicAdjustSlicing() defaults the minimum macroblock requirement per
slice (iMinimalMbNum) to the frame's macroblock width (1 row per slice).
For frames with extreme aspect ratios (such as very wide resolutions),
requiring 1 macroblock row per slice across multiple slices can exceed the
total number of macroblocks in the frame.

This change ensures robust macroblock distribution and load balancing by:

• Falling back iMinimalMbNum to 1 macroblock per slice when row-based
  heuristics exceed frame capacity in RC_OFF_MODE.
• Skipping adjustment and logging a warning when the requested number of
  slices equals or exceeds the total macroblocks in the frame.
• Adding a capacity guard in GomValidCheckSliceMbNum() for rate-controlled
  encoding modes.
• Adding a unit test (DynamicAdjustSlicingExtremeAspectRatio) to verify
  dynamic slice adjustment and load balancing on wide aspect ratio frames.