Add config option to allow EGL usage when built with MMAL

app/app.pro

@@ -286,6 +286,18 @@ mmal {
     DEFINES += HAVE_MMAL
     SOURCES += streaming/video/ffmpeg-renderers/mmal.cpp
     HEADERS += streaming/video/ffmpeg-renderers/mmal.h
+
+    # We suppress EGL usage when MMAL is available because MMAL has
+    # significantly better performance than EGL on the Pi. Setting
+    # this option allows EGL usage even if built with MMAL support.
+    #
+    # It is highly recommended to also build with 'glslow' to avoid
+    # EGL being preferred if direct DRM rendering is available.
+    allow-egl-with-mmal {
+        message(Allowing EGL usage with MMAL enabled)
+
+        DEFINES += ALLOW_EGL_WITH_MMAL
+    }
 }
 libdrm {
     message(DRM renderer selected)

app/streaming/video/ffmpeg-renderers/drm.cpp

@@ -1239,7 +1239,7 @@ bool DrmRenderer::canExportEGL() {
         return false;
     }
 
-#ifdef HAVE_MMAL
+#if defined(HAVE_MMAL) && !defined(ALLOW_EGL_WITH_MMAL)
     // EGL rendering is so slow on the Raspberry Pi 4 that we should basically
     // never use it. It is suitable for 1080p 30 FPS on a good day, and much
     // much less than that if you decide to do something crazy like stream

app/streaming/video/ffmpeg.cpp

@@ -1032,7 +1032,7 @@ bool FFmpegVideoDecoder::isDecoderIgnored(const AVCodec *decoder)
 {
     Q_UNUSED(decoder);
 
-#ifdef HAVE_MMAL
+#if defined(HAVE_MMAL) && !defined(ALLOW_EGL_WITH_MMAL)
     // Only enable V4L2M2M by default on non-MMAL (RPi) builds. The performance
     // of the V4L2M2M wrapper around MMAL is not enough for 1080p 60 FPS, so we
     // would rather show the missing hardware acceleration warning when the user