Blend OSPRay buffer using floating point texture

Improve OSPRay performance to avoid costly CPU conversion from float framebuffer to unsigned char texture.
It is now using a floating point texture to copy values to the OpenGL framebuffer (OpenGL takes care of the conversion).
It is also now possible to use render pass using color values > 1 (like vtkToneMappingPas) on the OSPRay result.
When not using an OpenGL context, we are using WriteLayer method, the conversion should be done here.

backport: release