Fix GL_INVALID_ENUM error in imgui_impl_opengl3 when calling glGetIntegerv on GL_CLIP_ORIGIN

[s7jones]

On OpenGL 3.20 with GLEW I have found that GL_CLIP_ORIGIN is defined and therefore the code block is run but it is invalid to call glGetIntegerv on GL_CLIP_ORIGIN in OpenGL 3.20 (Intel HD 4000).

The resulting invalid call causes a GL_INVALID_ENUM to be returned.

This fix adds a runtime check to ensure that we're using OpenGL 4.5 before using the clip control functionality.

[ocornut]

Hello @s7jones, thanks for the PR and pointing out to this issue.
AFAIK you haven't tested this as bool clip_origin_lower_left is now declared in a scope which is inaccessibel to its other uses, so code won't compile if GL_CLIP_ORIGIN is defined.

Additionally, I feel like if we test for g_GlVersion >= 450 we might be able to remove the __APPLE__ test.

[ocornut]

Reworked as 71dda21 and merged, thank you!

[s7jones]

Hi @ocornut.

bool clip_origin_lower_left is now declared in a scope which is inaccessibel to its other uses

Sorry about that, I had tested on my local conan setup and then transcribed the changes to master - but didn't spot that.

Thanks for the merge.