fix: ID-3404: JOGL renderer and off-screen view do not dispose all resources properly

[meinders]

Several issues are fixed that greatly reduce the amount of resources leaked during off-screen rendering:

• JOGLOffscreenView doesn't dispose JOGLRenderer at all
• JOGLRenderer does not dispose ShadowMap
• ShadowMap does not dispose the textures it creates
• JOGLRenderer does not dispose of its accumulation buffers, which hold texture objects
• TextureCache removes texture proxies for valid textures when context is re-used
• ShaderManager should detach shaders from the program after linking

Unfortunately it's not perfect yet. But that may or may not be an issue with this library.

[meinders]

(I'll make a follow-up PR if there are any serious issues.)