external: add a glGetBufferSubData() entrypoint on WebGL 2.

Yay, I can abuse/reuse the features I had to add to flextGL because of
GOOGLE!!!, and with only one additional patch to support local xml
files I can very conveniently include whatever entrypoints I need.

src/MagnumExternal/OpenGL/GLES3/Emscripten/extensions.txt

@@ -5,6 +5,7 @@
 version 3.0 es
 
 extraspec https://raw.githubusercontent.com/google/angle/master/scripts/gl_angle_ext.xml
+extraspec webgl.xml
 
 extension EXT_texture_filter_anisotropic        optional
 # It's actually EXT_disjoint_timer_query_webgl2, but that's not known to gl.xml
@@ -35,6 +36,8 @@ extension ANGLE_base_vertex_base_instance       optional
 # fortunately ES3 defines these constants on its own so I don't need it anyway.
 # extension ANGLE_compressed_texture_etc        optional
 
+extension MAGNUM_what_webgl_has_but_es_not      optional
+
 begin functions blacklist
     # Not present in WEBGL_blend_equation_advanced_coherent
     BlendBarrierKHR

src/MagnumExternal/OpenGL/GLES3/Emscripten/webgl.xml

@@ -0,0 +1,24 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<registry>
+    <comment>
+        This contains signatures of functions that are in WebGL 2 but not in ES
+        and (of course) gl.xml doesn't bother adding any WebGL-specific special
+        case.
+    </comment>
+
+    <!-- SECTION: GL command definitions. -->
+   <commands namespace="GL">
+        <!-- glGetBufferSubData is defined in gl.xml already, so nothing to do
+             here. It only needs to be required from some extension that's then
+             listed in extensions.txt. -->
+    </commands>
+
+    <!-- SECTION: MAGNUM extension interface definitions -->
+    <extensions>
+        <extension name="GL_MAGNUM_what_webgl_has_but_es_not" supported="gles2">
+            <require>
+                <command name="glGetBufferSubData"/>
+            </require>
+        </extension>
+    </extensions>
+</registry>

src/MagnumExternal/OpenGL/GLES3/flextGLEmscripten.h

@@ -1173,6 +1173,10 @@ GLAPI void glDisableiEXT(GLenum, GLuint);
 GLAPI void glEnableiEXT(GLenum, GLuint);
 GLAPI GLboolean glIsEnablediEXT(GLenum, GLuint);
 
+/* GL_MAGNUM_what_webgl_has_but_es_not */
+
+GLAPI void glGetBufferSubData(GLenum, GLintptr, GLsizeiptr, void *);
+
 /* GL_OVR_multiview */
 
 GLAPI void glFramebufferTextureMultiviewOVR(GLenum, GLenum, GLuint, GLint, GLint, GLsizei);