doc/pages/gfx/shader_def.dox: added group stub.
tool_build/resource_compiler/shader_def: specify unit type by string, not named node.¹

1: To avoid /more/ indentation of shader source and to ensure there
is no misconception that multiple shader units can occur in the same
shader definition. Different definitions per-renderer should use the
renderer name tag in the resource identity.
gfx/renderer/opengl: tidy.
gfx: added ParamBlockBinding.
gfx/renderer: added map_buffer() variant for ParamBlockBinding.