Changed default resolution behavior to use environment variable width and height if set instead of hard coded 1024x768

[DenTechs]

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• I have read the contributing guideline and understand that I have made the correct modifications

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Description:

Changed the default resolution behavior to use environment variable width and height if set instead of the currently hard coded 1024x768, modeline is generated using CVT.

Benefits of this PR and context:

When a manual resolution is set, instead of defaulting to 1024x768 and then changing on first connect, the manual resolution is used immediately. Additionally this change prevents a bug in my firefox ffmpeg capture fork as it can't handle the resolution changing dynamically.

How Has This Been Tested?

Built and tested with my fork of firefox and confirmed that the resolution was set to the one provided by the environment file instead of the hard coded 1024x768

This change should not have any affects on other areas of the code.

Source / References:

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[LinuxServer-CI]

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https://ci-tests.linuxserver.io/lspipepr/selkies-base/ubuntunoble-3620affb-pkg-3620affb-dev-61dcef89f07c1b0728f96d0fccdce980ccda7090-pr-113/index.html
https://ci-tests.linuxserver.io/lspipepr/selkies-base/ubuntunoble-3620affb-pkg-3620affb-dev-61dcef89f07c1b0728f96d0fccdce980ccda7090-pr-113/shellcheck-result.xml

Tag	Passed
amd64-ubuntunoble-3620affb-pkg-3620affb-dev-61dcef89f07c1b0728f96d0fccdce980ccda7090-pr-113	✅
arm64v8-ubuntunoble-3620affb-pkg-3620affb-dev-61dcef89f07c1b0728f96d0fccdce980ccda7090-pr-113	✅

[LinuxServer-CI]

I am a bot, here are the test results for this PR:
https://ci-tests.linuxserver.io/lspipepr/selkies-base/ubuntunoble-a671365e-pkg-a671365e-dev-e29a081e208330264405e8e0958e5f2b9bc5f9d0-pr-113/index.html
https://ci-tests.linuxserver.io/lspipepr/selkies-base/ubuntunoble-a671365e-pkg-a671365e-dev-e29a081e208330264405e8e0958e5f2b9bc5f9d0-pr-113/shellcheck-result.xml

Tag	Passed
amd64-ubuntunoble-a671365e-pkg-a671365e-dev-e29a081e208330264405e8e0958e5f2b9bc5f9d0-pr-113	✅
arm64v8-ubuntunoble-a671365e-pkg-a671365e-dev-e29a081e208330264405e8e0958e5f2b9bc5f9d0-pr-113	✅