GL: don't treat NV_depth_buffer_float as being core in GL 3.0.

The glDepthRangedNV() and glClearDepthdNV() behave differently from
glDepthRange() and glClearDepth() -- they don't clamp. There doesn't
seem to be any change in recent GL related to this, so this extension is
still valuable for improving depth buffer precision. Or, alternatively,
it's possible to use the glClipControl() API from GL 4.5 to achieve the
same.

src/Magnum/GL/Context.cpp

@@ -110,6 +110,7 @@ constexpr Extension ExtensionList[]{
     Extensions::KHR::texture_compression_astc_hdr{},
     Extensions::KHR::texture_compression_astc_ldr{},
     Extensions::KHR::texture_compression_astc_sliced_3d{},
+    Extensions::NV::depth_buffer_float{},
     Extensions::NV::fragment_shader_barycentric{},
     Extensions::NV::sample_locations{},
     Extensions::OVR::multiview{},
@@ -135,8 +136,7 @@ constexpr Extension ExtensionList300[]{
     Extensions::EXT::texture_shared_exponent{},
     Extensions::EXT::transform_feedback{},
     Extensions::MAGNUM::shader_vertex_id{},
-    Extensions::NV::conditional_render{},
-    Extensions::NV::depth_buffer_float{}
+    Extensions::NV::conditional_render{}
 };
 constexpr Extension ExtensionList310[]{
     Extensions::ARB::copy_buffer{},

src/Magnum/GL/Extensions.h

@@ -261,7 +261,7 @@ namespace AMD {
     _extension(171,MAGNUM,shader_vertex_id,             GL300, GL300)
 } namespace NV {
     _extension(175,NV,primitive_restart,                GL210, GL310) // #285
-    _extension(176,NV,depth_buffer_float,               GL210, GL300) // #334
+    _extension(176,NV,depth_buffer_float,               GL210,  None) // #334
     _extension(177,NV,conditional_render,               GL210, GL300) // #346
     /* NV_draw_texture not supported */                               // #430
     _extension(178,NV,sample_locations,                 GL210,  None) // #472