Allow the use of builtin bit functions from ES 3.1

[wizgrav]

I'm working on a project where the bulk of work is traversing hierarchies of bitmasks in u32s and having the ability to use hardware instructions to find the LSB MSB and count the set bits would help a lot. Could we bring those functions from ES 3.1 (bitCount, findLSB, findMSB) as an extension to WebGL 2?

[kdashg]

Yeah, this sounds pretty easy to add.

[lexaknyazev]

FWIW, all math ops (integer as well as a few others) introduced in GLSL ES 3.10 are equally easy to add since they are already supported in the translator:

• Integer Functions

  • bitfieldExtract and bitfieldInsert
  • bitfieldReverse
  • bitCount
  • findLSB and findMSB
  • uaddCarry and usubBorrow
  • umulExtended and imulExtended

• Floating-Point Pack and Unpack Functions

  • [un]packUnorm4x8
  • [un]packSnorm4x8

• Common Functions

  • mix with genIType/genUType/genBType
  • frexp and ldexp

[wizgrav]

@lexaknyazev yeah probably best to pack as many functions as possible in one extension to avoid polluting the registry