[No squash] Corrections to entity and node light calculation

[x2048]

This PR fixes two things:

1. When smooth light is disabled, makes visible brightness of nodebox and other draw types the same as NDT_SOLID at the same light level:
   
2. Corrects computation of brightness for entity glow property to work on the light levels (0..LIGHT_MAX):
   a. Glow is added to the map light level at the node position, before converting to the color space
   b. Additional brightness is added at the final stage in the same was as for light sources

The reason for a single PR is that light calculation for both scenarios is based on getInteriorLight function

Fixes #12686 in a formally correct way.

To do

This PR is Ready for Review.

How to test

In devtest:
Test 1:

1. Throw various light sources on the ground in a dark place.
2. Brightness of the item entities must reflect the light source level

Note, depending on your hardware, dim light sources (1-3) may not be visible in full darkness.

Test 2:

1. Place arrays of sandstone, sandstone slab and sandstone stairs in sunlight and check that the brightness level is the same
2. Place arrays of sandstone, sandstone slab and sandstone stairs near an array of light sources as show below:
   
3. Brightness of stairs and slabs must be exactly one level higher than on solid nodes. This is because solid nodes use light levels of the air nodes above them, which are 1 manhattan unit further away from the light source (as displayed above).

[SmallJoker]

glow is a value between [0, LIGHT_SUN] but encode_light treats the increment parameter as a value between [0, 255] for the effective brightness, hence the decode_light call here was more correct than the current code.

[x2048]

I have added comments explaining how glow is used now:

• Glow now impacts light level of the entity in the getInteriorLight call
• A small boost is added to the final color to make entities brighter at the same light level

[Zughy]

@x2048 rebase needed

[SmallJoker]

LGTM