Backends: OpenGL3: Backup and restore GL_PIXEL_UNPACK_BUFFER

[clementgallet]

When creating the font texture with glTexImage2D(), the result is affected by the buffer bound to the GL_PIXEL_UNPACK_BUFFER target. This was causing the following error:

GL error 0x502 returned from 'glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA, width, height, 0, GL_RGBA, GL_UNSIGNED_BYTE, pixels)'.

In the PR, I'm backing up the bound buffer, unbinds the buffer and restoring it at the end. This fixes blank text when hooking Dear ImGui into Yuzu emulator:

[ocornut]

Thank you for your PR.
You could find which GL context version this is going to require, and if this will compile with GL ES 2 / ES 3 / WebGL, so can add suitable runtime and/or compile-time checks if necessary?

[ocornut]

According to https://registry.khronos.org/OpenGL/api/GL/glcorearb.h, GL_PIXEL_UNPACK_BUFFER seem so to be a GL 2.1 thing. I will add the corresponding checks and merge this.

[ocornut]

Merged as 1ce41f6, thank you!