Fix the mess with WebGL 2 extensions.

EXT_sRGB is core in WebGL 2 and so it's not available as an extension,
while WEBGL_texture_compression_s3tc was accidentally left out for WebGL
2.

src/Magnum/Context.cpp

@@ -234,11 +234,12 @@ const std::vector<Extension>& Extension::extensions(Version version) {
     #elif defined(MAGNUM_TARGET_WEBGL)
     static const std::vector<Extension> extensions{
         _extension(GL,EXT,texture_filter_anisotropic),
-        _extension(GL,EXT,sRGB),
-        _extension(GL,EXT,disjoint_timer_query)};
+        _extension(GL,EXT,disjoint_timer_query),
+        _extension(GL,WEBGL,compressed_texture_s3tc)};
     #ifdef MAGNUM_TARGET_GLES2
     static const std::vector<Extension> extensionsES300{
         _extension(GL,ANGLE,instanced_arrays),
+        _extension(GL,EXT,sRGB),
         _extension(GL,EXT,blend_minmax),
         _extension(GL,EXT,shader_texture_lod),
         _extension(GL,OES,texture_float),
@@ -249,7 +250,6 @@ const std::vector<Extension>& Extension::extensions(Version version) {
         _extension(GL,OES,texture_float_linear),
         _extension(GL,OES,texture_half_float_linear),
         _extension(GL,OES,fbo_render_mipmap),
-        _extension(GL,WEBGL,compressed_texture_s3tc),
         _extension(GL,WEBGL,depth_texture),
         _extension(GL,WEBGL,draw_buffers)};
     #endif

src/Magnum/Extensions.h

@@ -249,8 +249,8 @@ namespace GL {
         #endif
     } namespace EXT {
         _extension(GL,EXT,texture_filter_anisotropic, GLES200,  None) // #11
-        _extension(GL,EXT,sRGB,                     GLES200,    None) // #17
         #ifdef MAGNUM_TARGET_GLES2
+        _extension(GL,EXT,sRGB,                     GLES200, GLES300) // #17
         _extension(GL,EXT,blend_minmax,             GLES200, GLES300) // #25
         #endif
         _extension(GL,EXT,disjoint_timer_query,     GLES200,    None) // #26
@@ -269,8 +269,8 @@ namespace GL {
         _extension(GL,OES,fbo_render_mipmap,        GLES200, GLES300) // #28
         #endif
     } namespace WEBGL {
-        #ifdef MAGNUM_TARGET_GLES2
         _extension(GL,WEBGL,compressed_texture_s3tc, GLES200,   None) // #8
+        #ifdef MAGNUM_TARGET_GLES2
         _extension(GL,WEBGL,depth_texture,          GLES200, GLES300) // #9
         _extension(GL,WEBGL,draw_buffers,           GLES200, GLES300) // #18
         #endif