doc: GL mapping cleanup, separated the table by letters.
doc: added forgotten glHint() to mapping table.

There are probably more.
doc: added upcoming glGetQueryBufferObject() to mapping.

Added as bugfix to ARB_direct_state_access (GL 4.5), mentioned in
https://twitter.com/piers_daniell/status/510106059348770817.
doc: cleaned up GL support table to have the col separator at tab stop.
doc: added GL_MAX_VERTEX_ATTRIB_STRIDE to mapping and support table.

Added in 4.4 but not backed by any extension.
doc: removed deprecated GL features from the mapping table.
doc: explicitly mention that we support all GLSL versions.

The support is just a matter of adding a value to switch in Shader.cpp,
but even that must be done by someone.
Replaced NV_half_float with hopefully more standard ARB_half_float.

According to reports on delphigl.de this extension is far more supported
in comparison to the NV version (and also there's much less FF cruft in
the specification).
doc: cleanup.
Added remaining non-core ARB/KHR extensions to the list.
Support for ARB_robustness_isolation.

Easier than forgetting about what I read and then learning that all
again later.
Replaced EXT_framebuffer_sRGB with ARB_framebuffer_sRGB.

Also seems to have wider support than the EXT version.
Support ARB_framebuffer_sRGB.

Hell, this extension (and EXT_sRGB on ES) is written in so complicated
language that I'm not sure at all if I understood it right and
implemented completely.