Make filter Context nullability explicit

[blihio]

Summary

CropFilterRender accepted a nullable Kotlin Context while passing it to Java code that dereferences the context as non-null. This change makes the nullability contract explicit across the filter initialization path while preserving existing behavior for valid calls.

Changes

• Add @NonNull to the Java filter initialization parameters.
• Add @NonNull to GlUtil.getStringFromRaw.
• Change CropFilterRender.initGlFilter from Context? to Context.
• Change MainRender.context to a non-null lateinit property.
• Add a regression test covering non-null context forwarding.

Validation

.\gradlew.bat :encoder:testDebugUnitTest

The build completed successfully:
BUILD SUCCESSFUL

The regression test BaseFilterRenderTest.initGlForwardsContextToFilter passed.
Related to #2207

[blihio]

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