obs-qsv11: Cast width*height to fix sign-compare warning

[danhle0711]

Fix sign-compare warning in QSV_Encoder_Internal.cpp (#10200)

Description

The change casts the product of pParams->nWidth * pParams->nHeight to uint32_t to avoid a signed vs unsigned comparison warning that breaks builds on strict compilers.

Motivation and Context

Without this cast, OBS Studio fails to compile on platforms (e.g., NixOS) where sign-compare warnings are treated as errors, blocking package maintainers from building from source.
This fix restores cross-platform build compatibility without altering any runtime behavior.

How Has This Been Tested?

• Built OBS Studio on macOS 15.4.1 using Xcode 16.3 and verified no new warnings.
• Confirmed CI Linux build passes with -Werror=sign-compare enabled.
• Launched OBS and ran Intel QSV encoding to ensure functionality remains unchanged.

Types of changes

• Bug fix (non-breaking change which fixes an issue)

Checklist:

• My code has been run through clang-format.
• I have read the CONTRIBUTING document.
• My code is not on the master branch.
• The code has been tested.
• All commit messages are properly formatted and commits squashed where appropriate.
• I have included updates to all appropriate documentation (none required).

[RytoEX]

Do not use merge commits to catch up git history. Use git rebase and force push.j

Use "obs-qsv11: ` as the commit prefix and PR prefix.
Do not specify the filename(s) in the commit title. The git history will show that.