Structs and fragment-to-light varyings for 3.4

[lyuma]

Backport of the following PRs by @Chaosus to 3.x, adding support for structs and the ability to declare varyings between fragment and light functions:

All changes support GLES2 and GLES3.

#35249 Implementation of struct for shaders
#36141 Added support for arrays as shader struct members
#44705 Fix using post-init shader array constructors
#44698 Allow passing varying from fragment to light shader function
#36251 Fix shader crash if duplicated struct members created
#46272 Prevents shader crash if passing invalid struct to the return statement
#46275 Prevents shader crash if two struct with the same name are declared

[Chaosus]

@lyuma Great! But there are some post-release patches for structs maded by me:
#36251
#46272
#46275
If you could merge the changes from them that would be perfect.

[lyuma]

Sure thing. I also see a couple other small bugfixes in the commit history. I'll go through them tomorrow. Luckily they all seem small and easy to merge.

[lyuma]

@Chaosus I have backported the remaining three changes from the set, and updated it to accommodate the recent reformatting update from last week.

[Chaosus]

Great, and thank you, @lyuma. Unfortunaly I'm unable to check it up on my PC properly now (I'm in hospital) but I trust you and approve it.

[akien-mga]

Thanks!