[WIP] Added an option to preview generated GLSL code to shader editor
This PR allows seeing generated code directly within shader editor. I've implemented it maximally gently - only one new function shader_get_gen_code
to VisualServer, no string
buffers for code - only simple call of glGetShaderCode...
Of course, currently, it lacks some features like a search bar, but I need to know whether is this change is desired before continue. Closes #1326