opengl: expose API to transform coords in CPU

All coordinates transforms were applied from the fragment shader generated by the sampler. This simplified the API a lot: the filters could just let the generated sampler code manage all the transforms.

However, this is wasteful, since all these transforms are linear.

Instead, expose an API to let the filters transform their coordinates from CPU or vertex shader.

This change has been made possible by !624 (merged) merged recently.

Refs discussion #26066 (comment 280411)