Leave GL_FRAMEBUFFER_SRGB disabled by default

[szabolcsdombi]

Despite it is recommended to keep this enabled, it breaks integrations like imgui rendering to the default framebuffer.
Also it affects the glBlitFramebuffers in an odd way copying between non srgb and srgb images.
With the zengl examples it is more common to disable it for the blit than to actually use it while enabled.
Some drivers are not supporting GL_FRAMEBUFFER_SRGB according to the specs.

ZenGL should enable GL_FRAMEBUFFER_SRGB when needed and disable it afterwards.
This change should not affect existing users.

[szabolcsdombi]

blit from an srgb image to an srgb screen

[szabolcsdombi]

Moving this call to be enabled in the render command is expensive.
Also blit requires a more cumbersome logic to set this variable global parameter to the right value.

Having this flag enabled globally is indeed the right way to do it.