It means '[The small amount of light that there was] struggled through...' It's a very neat, condensed way of combining two ideas: 1. There wasn't much light 2. This small amount of light struggled through the unwashed front window and then gave up the ghost

Hi 牛牛! In this sentence, the "what" at the beginning is a poetic way of writing "the...that." In other words, we can rewrite the beginning of this sentence to read "The light that struggled through..." and have it mean the same thing. I hope I've answered your question. If not, please let me know.