docs: document that GL_ARB_get_texture_sub_image is completed

Reviewed-by: Ilia Mirkin <imirkin@alum.mit.edu>

docs/relnotes/10.7.0.html

@@ -47,6 +47,7 @@ Note: some of the new features are only available with certain drivers.
 <li>GL_AMD_vertex_shader_viewport_index on radeonsi</li>
 <li>GL_ARB_fragment_layer_viewport on radeonsi</li>
 <li>GL_ARB_framebuffer_no_attachments on i965</li>
+<li>GL_ARB_get_texture_sub_image for all drivers</li>
 <li>GL_ARB_gpu_shader_fp64 on llvmpipe, radeonsi</li>
 <li>GL_ARB_shader_stencil_export on llvmpipe</li>
 <li>GL_ARB_vertex_attrib_64bit on llvmpipe, radeonsi</li>