Fix transparent viewport backgrounds with custom clear color

[LRFLEW]

This fixes #79778. This code change zeroes out the alpha component of the clear color. This basically just ports a change made in #61109 from the Forward+ renderer to the Mobile renderer, as well as an equivalent change to the Compatibility renderer.

[clayjohn]

Looks great!

If you want you can do the same fix for OpenGL (either in this PR or a followup). Clear color is set here:

godot/drivers/gles3/rasterizer_scene_gles3.cpp

Lines 1982 to 1984 in 0c2144d

	if (!keep_color) {
	glClearBufferfv(GL_COLOR, 0, clear_color.components);
	}

[LRFLEW]

I went ahead and made the change to the PR for the Compatibility renderer. I tested it, and it is working, but with a caveat: The viewport's transparency doesn't seem to update until you reload the scene. I'm not sure why, but I'll look into it.

[LRFLEW]

For the Compatibility renderer, I changed if (render_data.transparent_bg) to if (rt->is_transparent), which resolved the issue of the background transparency not updating until the scene was reloaded. I don't really understand why the two are different, but the change worked. EDIT: Is this a bug? It doesn't make sense to me that these would be different, and I'm starting to think this may be a latent bug that I brought out. I can try looking into it a bit. EDIT 2: Seems like what's happening is that RenderSceneBuffersGLES3::configure is what sets rb->is_transparent, but it isn't called from RendererViewport::viewport_set_transparent_background, so it doesn't update if the viewport's background transparency is updated. It doesn't seem like a trivial fix, though, as calling configure from there requires specifying a lot of arguments that aren't necessarily available, and I'm wondering if we really need rb->is_transparent at all if we have rt->is_transparent.

I also made another change to force consistency between the renderers. Right now, setting an alpha on the custom clear color is ignored by the Forward+ and Mobile renderers. However, the alpha of the clear color was used when clearing the viewport in the Compatibility renderer, even with the viewport set to use an opaque background. I decided to force the alpha of the clear color to be 1 when the render target is not transparent to make the Compatibility renderer's behavior match the other renderers.

[LRFLEW]

From what I can tell, RenderSceneBuffersGLES3::is_transparent is redundant, since it's just duplicating GLES3::RenderTarget::is_transparent, and trying to keep them in sync looks to be a problem. I added a commit (not an amended commit since it might want to be approved / declined separately, and wanted to keep an easy way to undo it) that removes RenderSceneBuffersGLES3::is_transparent entirely, which turned out to be fairly simple. This directly addresses the issue I had when making this work with the Compatibility renderer, as well as addressing any other parts of the Compatibility renderer that depended on render_data.transparent_bg being accurate.

[clayjohn]

From what I can tell, RenderSceneBuffersGLES3::is_transparent is redundant, since it's just duplicating GLES3::RenderTarget::is_transparent, and trying to keep them in sync looks to be a problem. I added a commit (not an amended commit since it might want to be approved / declined separately, and wanted to keep an easy way to undo it) that removes RenderSceneBuffersGLES3::is_transparent entirely, which turned out to be fairly simple. This directly addresses the issue I had when making this work with the Compatibility renderer, as well as addressing any other parts of the Compatibility renderer that depended on render_data.transparent_bg being accurate.

Thanks for looking into that more deeply. Indeed it makes sense to remove is_transparent from render buffers. That was an artifact from before we merged RT and RB.

Go ahead and squash the commits together, then we can merge this!

[YuriSizov]

Also needs a rebase now :)

[YuriSizov]

Thanks!

[YuriSizov]

Cherry-picked for 4.1.4.