Blender Git Loki

Immediate Mode: replacing glPushAttrib/glPopAttrib

Reference document: http://docs.gl/gl3/glPushAttrib

This patch only tackles the bits that are set by Blender with the
following exceptions:

1) Deprecated states (e.g., GL_STIPPLE) are not saved/restored

2) The exception being GL_ALPHA_TEST, which will be removed, but it may
affect drawing too much now. To be removed once we no longer set GL_ALPHA_TEST
elsewhere.

3) paint_cursor will be tackled separated, since it was abusing
glPush/PopAttrib in the first place.

4) Despite what the glPushAttrib page above may suggest, GL_DEPTH_WRITEMASK needs glGet, not glIsEnabled

5) BGE is still a problem since it relies on GL_ALL_ATTRIB_BITS which
would lead to a way more complete/lenghty solution. Since the BGE has
other (OpenGL deprecated) problems anyways, it can be handled on its own
time.

Finally, the original design for 2.8 was to implement a proper stack
system. However we need to move to core profile sooner than later. So
this is a pragmatic temporary (that may be permanent) solution.

Reviewers: merwin, campbellbarton

Differential Revision: https://developer.blender.org/D2600