Fixed OpenGLShader::PreProcess Logic (issue #120)

[Puddlestomper]

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name: 'Pull Request: Implemented issue'
about: Fixes and explanatory comments for OpenGLShader::PreProcess
title: Fixed OpenGLShader::PreProcess logic (issue #120)
labels: ''
assignees: ''

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PR impact (Make sure to add closing keywords)

List of related issues/PRs this will solve:

Impact	Issue/PR
Relevant issue	Resolves #120
Other issues this solves	None
Other PRs this solves	None

Proposed fix (Make sure you've read on how to contribute to Hazel)

We add two steps:

1. Assert that nextLinePos is not std::string::npos.
2. Split the final assign operation to let the substring run to the end of the file if pos is std::string::npos (if there is no next #type token).

We also add comments to clarify the purpose of each variable.

Additional context

This compiles and runs without error on the latest commit of Hazel.