[Xbox] DX fixes and improvements

[thexai]

Description

• Use 3 back buffers on Xbox as 6 causes video not fluid all the time (jerks every 10 seconds approx.).
• Use memory alignment of 128 bytes for DXVA2 decoding surfaces, same as Windows desktop.
• Removed workaround code for limited memory as is not actually needed. Current code never uses more of 16 decoding surfaces on Xbox.

Motivation and context

I have had the opportunity to do tests on an Xbox series S and these changes are necessary for perfectly smooth video playback (4K HEVC up to 60 fps).

How has this been tested?

Runtime tested on Xbox series S

What is the effect on users?

Smooth playback of 4K HEVC 23.976/59.94 fps using DXVA2 hardware acceleration.

Types of change

• Bug fix (non-breaking change which fixes an issue)
• Clean up (non-breaking change which removes non-working, unmaintained functionality)
• Improvement (non-breaking change which improves existing functionality)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that will cause existing functionality to change)
• Cosmetic change (non-breaking change that doesn't touch code)
• None of the above (please explain below)

Checklist:

• My code follows the Code Guidelines of this project
• My change requires a change to the documentation, either Doxygen or wiki
• I have updated the documentation accordingly
• I have read the Contributing document
• I have added tests to cover my change
• All new and existing tests passed