build: Allow custom screenshot impl for downstream SDKs

[jpnurmi]

Allow downstream SDKs to select SENTRY_SCREENSHOT=custom so sentry-native skips the built-in screenshot capture source and expects an implementation from the embedding build.

[cursor]

Cursor Bugbot has reviewed your changes and found 2 potential issues.

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[cursor]

Missing WIN32 guard for windows screenshot option

Low Severity

The SENTRY_SCREENSHOT "windows" branch doesn't validate that it's actually running on WIN32, unlike the analogous SENTRY_TRANSPORT "winhttp" check which includes if(NOT WIN32) message(FATAL_ERROR ...). If a user explicitly passes -DSENTRY_SCREENSHOT=windows on a non-Windows platform, it would attempt to compile sentry_screenshot_windows.c with Windows-specific APIs, leading to confusing compilation errors rather than a clear configuration-time error.

 

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