Artifacts in water reflection due to imprecise screen-space coordinate computation

When positioning the camera right above the water surface, there are weird artifacts in the reflection:

(No, those are not micro-ripples 😉 )

Operating system, OpenMW version, etc.
OpenMW version is master, OS is Fedora 33, GPU/Mesa version is Radeon RX 560 Series (POLARIS11, DRM 3.42.0, 5.14.13-100.fc33.x86_64, LLVM 11.0.0) (0x67ff).

Root cause
The root cause of this are numerical issues in the reconstruction of screen-space coordinates; when the viewing direction is parallel to the water surface, and the camera is just above the surface, the screen-space projected location of the water quad can take on extreme values. water_vertex.glsl attaches those projected locations to the vertex locations.

water_fragment.glsl then accesses the interpolated information attached to the vertices; at some point in this process funny numerical shenanigans happen.

Potential solution
Use gl_FragCoord directly to access the screen-space coordinates. This makes the vertex shader unnecessary. Problem with this is that the viewport size needs to be passed to water_fragment.glsl. While this is simple in principle, and I'll open MR that kind of does this, OSG makes this frustratingly difficult. Viewport coordinates are also required by PostProcessing shaders; (not directly, see below) maybe there should be some unified way to access those (maybe using the SharedUniformStateUpdater)?