refactor plugin to use OpenGL fbo rendering

[mbaetgen-wup]

Rendering improvements

• use OpenGL render-to-texture with Frame Buffer Objects (FBO)
• send GL textures downstream - video/x-raw(memory:GLMemory)
• use buffer pool for managing GL textures
• optimized GL thread usage
• real-time rendering optimizations and real-time QoS
• offload rendering tasks from chain function to render buffer (sync and async rendering)

General fixes and improvements

• improved GST event handling (segments, QoS)
• fix some locking issues and race conditions
• fix sync issues with pipeline clock
• auto-detect offline or real-time rendering
• re-usable projectM/gst integration functions
• naming cleanup
• more docs

Requires projectM 4.2 (not yet released, current master branch version)

Test status
Linux: Real-time rendering stable (over 19h) / Offline rendering stable
Windows: ?

[rootnotez]

Does the /examples directory need to be created? It is confusing with the /test directory, and makes it confusing where the audio/video samples are kept.

[mbaetgen-wup]

that's a good point. The example itself may also not be that useful in it's current state. I will remove it from this PR and add it again in a separate PR later.