Basic fast filtering implementation

[clayjohn]

This PR implements the fast filtering algorithm described here.

Fast filtering uses a two pass approach to blur environment maps to create radiance maps. It has incredible speed benefits over traditional importance sampling but comes at the cost of quality in certain scenarios. The implementation in this PR is both faster and higher quality than our current implementation of importance sampling, however, it can only be used to generate a 128x128 radiance map. So the current importance sampling implementation is left in place so users can have crisper environment reflections if they need.

comparison of fast filtering (top, 10ms) and importance sampling (below, 75ms)

I have left the options structure more or less the same. Because there is no longer a low quality importance sampling I removed the "realtime_ggx" project setting and added a setting to choose the quality to use for the fast filtering algorithm. Low quality is default and provides excellent quality for most scenes. High quality can be used for use with especially high detail scenes.

Following this PR I would like to make another one improving the current importance sampling technique so that it is higher quality than the realtime option.

Speed Comparison

Current implementation using Importance Sampling

	High Quality	Low Quality
Array	75,000 us*	5,000 us*
Non-Array	20,000 us	670 us

Proposed implementation using Fast Filtering

	High Quality	Low Quality
Array	10,000 us*	4,000 us*
Non-Array	2,000 us	300 us

[akien-mga]

Clang warning:

servers/visual/rasterizer_rd/rasterizer_scene_rd.cpp:829:6: error: logical not is only applied to the left hand side of this comparison [-Werror,-Wlogical-not-parentheses]
        if (!storage->reflection_probe_get_update_mode(rpi->probe) == VS::REFLECTION_PROBE_UPDATE_ONCE) {
            ^                                                      ~~
servers/visual/rasterizer_rd/rasterizer_scene_rd.cpp:829:6: note: add parentheses after the '!' to evaluate the comparison first
        if (!storage->reflection_probe_get_update_mode(rpi->probe) == VS::REFLECTION_PROBE_UPDATE_ONCE) {
            ^
             (                                                                                        )
servers/visual/rasterizer_rd/rasterizer_scene_rd.cpp:829:6: note: add parentheses around left hand side expression to silence this warning
        if (!storage->reflection_probe_get_update_mode(rpi->probe) == VS::REFLECTION_PROBE_UPDATE_ONCE) {
            ^
            (                                                     )

[clayjohn]

@akien-mga Thanks!

Funny enough, that typo would have made the new code run 6 times every frame instead of once and I didn't even notice cause I still had > 60 FPS.

[clayjohn]

@akien-mga I will push a version with docs later today. Please don't merge until then.

[clayjohn]

@akien-mga Should be ready to merge pending the outcome of CI.
:)

[akien-mga]

Docs reviewed.

[clayjohn]

The build failure looks like an issue with the CI itself.

[akien-mga]

CI passed but there's a conflict with recent docs changes.

[clayjohn]

Fixed :)

[akien-mga]

Thanks!