ARB_sample_locations and AMD_shader_explicit_vertex_parameter /
NV_fragment_shader_barycentric for barycentic coords in the vertex
shader. The ARB extension mentions GLES but doesn't have any assigned
number, so I'm treating it as desktop-only. There's a NV variant that
does have an ES extension number, but I don't really want to add it just
because that.
This one explicitly loads GL 1.0 and 1.0 function pointers on EGL
contexts on NVidia drivers (Linux headless boxes), because somehow the
usual statically linked functions don't behave correctly.
The function pointer loading is now moved to Application classes to make
it possible to decide about platform-specific API at usage time, not at
library compilation time.
Currently it's not possible to create the Magnum context any other way
than through Application classes, will solve that in next commits.