fix(client_core): 🐛 Workaround for blank right eye view

[zmerp]

This is a workaround for a weird graphics bug. It seems the staging framebuffer cannot write to more than the size of the stream framebuffer, even if they have nothing to do with each other. The fix is to split the staging texture into two so each staging framebuffer has the same size as the stream framebuffers.

This bug has been investigated extensively .This is not a problem with shaders or viewports. The root cause is still unknown.

[The-personified-devil]

I'd still rather figure out the root cause as that could prevent future bugs, but debugging it seems hard and this should be good regardless.

[nowak-pl]

Root cause is enabled scissor test (https://www.khronos.org/opengl/wiki/Scissor_Test).
Add gl.disable(gl::SCISSOR_TEST); after

ALVR/alvr/client_core/src/graphics/staging.rs

Line 120 in 4d0d0ce

ck!(gl.viewport(0, 0, self.viewport_size.x, self.viewport_size.y));

or better yet store current state, disable and later reenable.

[zmerp]

@nowak-pl Ah, were you able to test that? Would you like to make a PR or should we do it? I will not revert this PR for now, as the changes here contribute for future support for sliced encoding.

[nowak-pl]

All I can say is simple disable works me - didn't test if something else breaks because it expects scissors enabled.