WIP: MRT Proof of concept

[Zyko0]

What issue is this addressing?

#2930

What this PR does | solves

This is a proof of concept of MRT (multiple render target), to investigate if the concept could be integrated to ebitengine, this is not a merge candidate for now.

go run ./examples/mrt

dsts = [4]*ebiten.Image{
		ebiten.NewImageWithOptions(image.Rect(0, 0, 128, 128), &ebiten.NewImageOptions{Unmanaged: true}),
		ebiten.NewImageWithOptions(image.Rect(0, 0, 128, 128), &ebiten.NewImageOptions{Unmanaged: true}),
		ebiten.NewImageWithOptions(image.Rect(0, 0, 128, 128), &ebiten.NewImageOptions{Unmanaged: true}),
		ebiten.NewImageWithOptions(image.Rect(0, 0, 128, 128), &ebiten.NewImageOptions{Unmanaged: true}),
}
shaderSrc = []byte(`
//kage:units pixels

package main

func Fragment(dst vec4, src vec2, color vec4) (vec4, vec4, vec4, vec4) {
	return vec4(1,0,0,1), vec4(0,1,0,1), vec4(0,0,1,1), vec4(1,0,1,1)
}
`

{
      ebiten.DrawTrianglesShaderMRT(dsts, vertices, indices, s, nil)
      // screen.Draw all 4 images to assess that they've been written to with the same geometry / draw call
}

• OpenGL
• DirectX 11
• DirectX 12
• WebGL => A bit more problematic due to the glsl compatibility between desktop
• Metal

Notes:

• It only seems to make sense with unmanaged *ebiten.Image (as in different internal textures).
  • dst images could be moved to separate atlases if they're not ebiten.unmanaged: Support Multiple render target (MRT) #2930 (comment)
  • dst images, if moved to new textures or existing (available) ones should share the same region (rectangle) as they originally do, because of scissor, viewport definition but mostly because the triangles define the logical destination position, moving to an arbitrary region would make the triangles target the wrong rectangle
  • dst images should have the same sizes probably, since the triangles, scissor and viewport should target the same region for all destination textures
• I set the max dst images count to 8, since 8 seems to be the guaranteed minimum by all drivers