[3.x] Image.FORMAT_R8 uses Alpha in GLES2 instead of Red channel, this is not reflected in docs

[Birdulon]

Godot version: v3.2.1.stable.official, though relevant code appears to be the same in master

OS/device including version:

Issue description: Using Image.FORMAT_R8 in GLES3 correctly produces ImageTextures that have your given data in the Red channel. In GLES2, the data is not in the Red channel but the Alpha channel, breaking compatibility between the two backends and also breaking expectations.
The documentation does not specify this behavior which can cause confusion.
Relevant GLES3 code:

godot/servers/rendering/rasterizer_rd/rasterizer_storage_rd.cpp

Lines 56 to 61 in 00949f0

	case Image::FORMAT_R8: {
	r_format.format = RD::DATA_FORMAT_R8_UNORM;
	r_format.swizzle_r = RD::TEXTURE_SWIZZLE_R;
	r_format.swizzle_g = RD::TEXTURE_SWIZZLE_ZERO;
	r_format.swizzle_b = RD::TEXTURE_SWIZZLE_ZERO;
	r_format.swizzle_a = RD::TEXTURE_SWIZZLE_ONE;

Relevant GLES2 code:

godot/drivers/gles2/rasterizer_storage_gles2.cpp

Lines 156 to 158 in 00949f0

	case Image::FORMAT_R8: {
	r_gl_internal_format = GL_ALPHA;
	r_gl_format = GL_ALPHA;

Relevant documentation: https://github.com/godotengine/godot/blob/master/doc/classes/Image.xml#L506-L507

Steps to reproduce: Create an Image with format Image.FORMAT_R8, add data to it either by drawing pixels or supplying it with an existing PoolByteArray, create an ImageTexture from that Image and draw it.

Minimal reproduction project:
Make a scene based off a Control, add a TextureRect as a child with the following script:

extends TextureRect

func _ready() -> void:
	rect_scale = Vector2(100, 100)
	var image = Image.new()
	image.create_from_data(3, 3, false, Image.FORMAT_R8, PoolByteArray([0, 20, 40, 60, 80, 100, 128, 180, 255]))
	var image_texture = ImageTexture.new()
	image_texture.create_from_image(image, 0)
	texture = image_texture

Proposed fixes:

1. Least disruptive to existing projects - add a note to the documentation specifying that Image.FORMAT_R8 will store data in the Alpha channel in GLES2.
2. Make the GLES2 backend use GL_LUMINANCE instead of GL_APLHA for the format and internal format. The OpenGL ES2.0 reference card states that the red channel is used to access the data in this format, which will ensure compatibility with godot shaders written for the GLES3 backend, and not break expectations for users who aren't already working around the current behavior. It won't draw the same pure-red gradient that the GLES3 backend might, but it will give more expected behavior for shader code.
   https://www.khronos.org/opengles/sdk/docs/reference_cards/OpenGL-ES-2_0-Reference-card.pdf
   

[clayjohn]

I would go with option 2. From what I can tell there is no difference in support between GL_LUMINANCE and GL_ALPHA. It looks like when porting over to GLES2 karroffel just went with GL_ALPHA because it seemed to be the closest to GL_RED

Note that GLES3 switches between GL_LUMINANCE and GL_RED for L8 depending on if mobile or desktop is used:

godot/drivers/gles3/rasterizer_storage_gles3.cpp

Lines 152 to 162 in 7ccb146

	case Image::FORMAT_L8: {
	#ifdef GLES_OVER_GL
	r_gl_internal_format = GL_R8;
	r_gl_format = GL_RED;
	r_gl_type = GL_UNSIGNED_BYTE;
	#else
	r_gl_internal_format = GL_LUMINANCE;
	r_gl_format = GL_LUMINANCE;
	r_gl_type = GL_UNSIGNED_BYTE;
	#endif
	} break;

[Calinou]

We can consider fixing this behavior in 3.5. Since 3.4 is nearing release and is in feature freeze, I think it's better to document the current status for now.