Initial glLoadGen work.

Added README, extension list, patch required for it to work on GL 2.1
and the generated files.

external/OpenGL/GL/README.md

@@ -0,0 +1,16 @@
+OpenGL header and extension loader is generated using [glLoadGen](https://bitbucket.org/alfonse/glloadgen),
+preferrably from tip of the development branch, currently `2.0`. For it to work
+flawlessly on OpenGL 2.1 and simultaenously not containing all that pre-core
+stuff, you need to patch it with included `glloadgen.patch`, which will then
+force it to use pre-3.0 `glGetString(GL_EXTENSIONS)` even if we request OpenGL
+4.4. Otherwise it would use `glGetStringi(GL_EXTENSIONS, i)`, which is not
+available on OpenGL 2.1 and extension loading will fail on these systems.
+
+See `extensions.txt` for a list of requested non-core extensions. Copy the file
+into glLoadGen directory and generate the files using Lua:
+
+    lua LoadGen.lua -style=pointer_c -indent=space -spec=gl -version=4.4 -profile=core -extfile=extensions.txt magnum
+
+Then copy generated `gl_magnum.c` and `gl_magnum.h` to this directory. You may
+want to remove trailing spaces to make the diff cleaner. Be sure to check the
+diff for suspicious changes.

external/OpenGL/GL/extensions.txt

@@ -0,0 +1,12 @@
+// The following output is taken from `magnum-info` with removed `GL_`
+// suffixes. Only vendor extensions are needed, everything else is pulled in
+// with OpenGL 4.4.
+
+    AMD_vertex_shader_layer
+    AMD_shader_trinary_minmax
+    ARB_robustness
+    ATI_texture_mirror_once
+    EXT_texture_filter_anisotropic
+    EXT_texture_mirror_clamp
+    EXT_direct_state_access
+    GREMEDY_string_marker

external/OpenGL/GL/glloadgen.patch

@@ -0,0 +1,17 @@
+diff --git a/modules/Specs.lua b/modules/Specs.lua
+index df4f5d5..95e0b97 100644
+--- a/modules/Specs.lua
++++ b/modules/Specs.lua
+@@ -116,9 +116,9 @@ function glx_spec.GetExtStringParamList(enumResolve) return "display, screen" en
+ --	Function name used to get an extension string.
+ --	Enumerator name used to get an extension string.
+ function gl_spec.GetIndexedExtStringFunc(options)
+-	if(tonumber(options.version) >= 3.0) then
+-		return {"GetIntegerv", "NUM_EXTENSIONS", "GetStringi", "EXTENSIONS"}
+-	end
++-- 	if(tonumber(options.version) >= 3.0) then
++-- 		return {"GetIntegerv", "NUM_EXTENSIONS", "GetStringi", "EXTENSIONS"}
++-- 	end
+ 	return nil
+ end
+ function wgl_spec.GetIndexedExtStringFunc(options) return nil end