Ensure MovieWriter output is in gamma space when using HDR 2D

[clayjohn]

Fixes: #81301
Fixes: #82351 (this is a documentation fix)

Right now when writing out movies they have to be in 8-bit sRGB space. So when using a linear render target we need to convert to gamma space before writing.

To support doing that I had to:

1. Expose viewport_is_using_hdr_2d. This is not exposed to users as its not a good idea to call this every frame (for Movie Writer we can't avoid constant GPU stalls anyway, so it's fine)
2. Add Image::linear_to_srgb() this is a direct copy of the existing Image::srgb_to_linear() function. (I generated the table of values using Color::linear_to_srgb())

Since this adds a new function let's wait until 4.4 to merge

[Calinou]

Expose viewport_is_using_hdr_2d. This is not exposed to users as its not a good idea to call this every frame (for Movie Writer we can't avoid constant GPU stalls anyway, so it's fine)

In the long term, we should investigate using some kind of ring buffer to avoid GPU stalls: godotengine/godot-proposals#4751
This could easily make Movie Maker mode twice as fast, particularly once #91263 is merged.

Perhaps HDR 2D status could be checked only once at startup to avoid constant stalls every frame. It's unlikely to change at runtime after all.

[clayjohn]

Perhaps HDR 2D status could be checked only once at startup to avoid constant stalls every frame. It's unlikely to change at runtime after all.

Unfortunately, the three function calls above it also force a rendering thread sync. So the addition of this check doesn't change anything

[Calinou]

Tested locally, it mostly works as expected.

There's a visible precision loss in the movie output compared to the window output (i.e. what I see in the window). Notice the glow's appearance in the top-left corner having noticeable banding, or even the lighting on the cube.

There's also a slight difference in overall brightness. The project uses Forward+ for reference.

Testing project: test_hdr_2d_movie.zip

Window output

Movie output before

Movie output after this PR

[clayjohn]

@Calinou I suspect the difference in output comes from the code we use to convert the RGBA16 texture to RGBA8 (it could be a rounding issue)

[akien-mga]

Thanks!