GL: fix and add a comment so I don't mess this up again in the future.

src/Magnum/GL/Context.cpp

@@ -245,7 +245,9 @@ constexpr Extension ExtensionList460[]{
 constexpr Extension ExtensionList[]{
     _extension(EXT,texture_filter_anisotropic),
     _extension(EXT,disjoint_timer_query),
+    #ifndef MAGNUM_TARGET_GLES2
     _extension(EXT,color_buffer_float),
+    #endif
     _extension(OES,texture_float_linear),
     _extension(WEBGL,compressed_texture_s3tc)};
 constexpr Extension ExtensionListES300[]{

src/Magnum/GL/Extensions.h

@@ -275,7 +275,10 @@ namespace ANGLE {
     #ifdef MAGNUM_TARGET_GLES2
     _extension( 7,EXT,shader_texture_lod,           GLES200, GLES300) // #27
     #endif
+    #ifndef MAGNUM_TARGET_GLES2
+    /* Replaces WEBGL_color_buffer_float from WebGL 1 */
     _extension( 8,EXT,color_buffer_float,           GLES300,    None) // #31
+    #endif
 } namespace OES {
     #ifdef MAGNUM_TARGET_GLES2
     _extension( 9,OES,texture_float,                GLES200, GLES300) // #1
@@ -293,6 +296,8 @@ namespace ANGLE {
     _extension(17,WEBGL,compressed_texture_s3tc,    GLES200,    None) // #8
     #ifdef MAGNUM_TARGET_GLES2
     _extension(18,WEBGL,depth_texture,              GLES200, GLES300) // #9
+    /* Subsumed by the EXT_color_buffer_float extension in WebGL 2, so
+       not exposing it on WebGL 2 builds even though it's not in core */
     _extension(19,WEBGL,color_buffer_float,         GLES200,    None) // #14
     _extension(20,WEBGL,draw_buffers,               GLES200, GLES300) // #18
     #endif

src/MagnumExternal/OpenGL/GLES2/Emscripten/extensions.txt

@@ -22,6 +22,6 @@ extension EXT_draw_buffers                      optional
 
 extension EXT_texture_filter_anisotropic        optional
 extension EXT_disjoint_timer_query              optional
-extension EXT_color_buffer_float                optional
 # These are used as a base for WEBGL_* extensions
+extension EXT_color_buffer_float                optional
 extension EXT_texture_compression_s3tc          optional