SCTE to OverlayEvent support

[lucabenvenuto]

Summary

This PR adds support for translating SCTE-35 SegmentationDescriptor events into DASH OverlayEvent elements, and unifies the previously separate scte-to-overlay and scte-to-alternative processing paths into a single always-on flow.

SCTE-35 → OverlayEvent mapping (ngx_morpheus_internal.cpp)

• New morph_overlay() function scans each Period for EventStreams with a scte35 schemeIdUri and converts them into a new urn:scte:dash:scte214-events EventStream.
• segmentationTypeId=56 (start) events read a segmentationUpidType=14 UPID whose value encodes the ad shape (shape=banner|skyscraper|lshape-right|lshape-left). Each shape maps to a hardcoded shape_cfg entry with URL, z-index, viewport, size, top-left position, and optional squeeze configuration.
• segmentationTypeId=57 (stop) events emit an OverlayEvent with mode=stop referencing the paired start event by refId.
• The original SCTE-35 EventStream is removed from the Period after conversion.

Unified processing flow

Previously, alternative MPD insertion (morph_alternative) was gated behind a ?mode=scte-to-alternative query parameter (HTTP module) and a -a CLI flag (morphdriver). Both gates have been removed. morph_alternative and the new morph_overlay now both run unconditionally on every morph_process call.

• morph_alternative now guards against claiming SCTE-35 streams that contain no SpliceInsert events, preventing it from consuming SegmentationDescriptor streams intended for overlay.
• morph_process signature simplified: const bool alternativeconf parameter dropped.
• ngx_http_morpheus_module.cpp, morpheus_main.cpp, and Makefile updated accordingly.

Docker image

Initial work was done on the feat/dockerimage branch (commit 74935c3). A multi-stage Dockerfile was added:

• Stage 1 (builder): starts from nginx:1.24.0, downloads matching nginx source, patches the build system to remove -Werror (required for C++ modules), fetches pugixml v1.15 source directly from the upstream repository, and compiles the dynamic module.
• Stage 2 (production): copies only the compiled .so into a clean nginx:1.24.0 image alongside the nginx config and MIME types. Creates /dev/shm/nginx/client_temp at container startup (tmpfs is not available at build time).