GL: don't include MAGNUM_shader_vertex_id on ES2 builds.

It makes one think such feature *could* be supported there, while it
couldn't.

doc/opengl-support.dox

@@ -397,7 +397,7 @@ Extension                                   | Status
 @gl_extension{EXT,draw_buffers}             | done
 @gl_extension{EXT,instanced_arrays}         | done
 @gl_extension{EXT,draw_instanced}           | done
-`MAGNUM_shader_vertex_id` \n @m_span{m-text m-dim} Pseudo-extension denoting support for the @glsl gl_VertexID @ce \n GLSL builtin. Exposed on ES3 builds as well. @m_endspan | done
+`MAGNUM_shader_vertex_id` \n @m_span{m-text m-dim} Pseudo-extension denoting support for the @glsl gl_VertexID @ce \n GLSL builtin. Exposed on ES3 builds only. @m_endspan | done
 @gl_extension{NV,draw_buffers}              | done
 @gl_extension{NV,fbo_color_attachments}     | done
 @gl_extension{NV,read_buffer}               | done
@@ -500,7 +500,7 @@ Extension                                   | Status
 @webgl_extension{EXT,color_buffer_half_float} | |
 @webgl_extension{EXT,blend_minmax}          | done
 @webgl_extension{EXT,shader_texture_lod}    | done (shading language only)
-`MAGNUM_shader_vertex_id` \n @m_span{m-text m-dim} Pseudo-extension denoting support for the @glsl gl_VertexID @ce \n GLSL builtin. Exposed on WebGL 2 builds as well. @m_endspan | done
+`MAGNUM_shader_vertex_id` \n @m_span{m-text m-dim} Pseudo-extension denoting support for the @glsl gl_VertexID @ce \n GLSL builtin. Exposed on WebGL 2 builds only. @m_endspan | done
 @webgl_extension{OES,texture_float}         | done
 @webgl_extension{OES,texture_half_float}    | done
 @webgl_extension{OES,standard_derivatives}  | done

src/Magnum/GL/Context.cpp

@@ -263,7 +263,9 @@ constexpr Extension ExtensionListES300[]{
     _extension(EXT,blend_minmax),
     _extension(EXT,shader_texture_lod),
     #endif
+    #ifndef MAGNUM_TARGET_GLES2
     _extension(MAGNUM,shader_vertex_id),
+    #endif
     #ifdef MAGNUM_TARGET_GLES2
     _extension(OES,texture_float),
     _extension(OES,texture_half_float),
@@ -348,7 +350,9 @@ constexpr Extension ExtensionListES300[]{
     _extension(EXT,instanced_arrays),
     _extension(EXT,draw_instanced),
     #endif
+    #ifndef MAGNUM_TARGET_GLES2
     _extension(MAGNUM,shader_vertex_id),
+    #endif
     #ifdef MAGNUM_TARGET_GLES2
     _extension(NV,draw_buffers),
     _extension(NV,fbo_color_attachments),

src/Magnum/GL/Extensions.h

@@ -309,7 +309,9 @@ namespace ANGLE {
     _extension(30,WEBGL,compressed_texture_astc,    GLES200,    None) // #30
     _extension(31,WEBGL,compressed_texture_s3tc_srgb, GLES200,  None) // #32
 } namespace MAGNUM {
+    #ifndef MAGNUM_TARGET_GLES2
     _extension(40,MAGNUM,shader_vertex_id,          GLES300, GLES300)
+    #endif
 }
 #else
 namespace ANDROID {
@@ -489,7 +491,9 @@ namespace ANDROID {
     _extension(126,OES,texture_storage_multisample_2d_array, GLES310, GLES320) // #174
     #endif
 } namespace MAGNUM {
+    #ifndef MAGNUM_TARGET_GLES2
     _extension(130,MAGNUM,shader_vertex_id,         GLES300, GLES300)
+    #endif
 }
 #endif
 #undef _extension

src/Magnum/GL/Test/ContextTest.cpp

@@ -29,6 +29,7 @@
 #include <Corrade/Utility/DebugStl.h>
 
 #include "Magnum/GL/Context.h"
+#include "Magnum/GL/Extensions.h"
 #include "Magnum/GL/Version.h"
 
 namespace Magnum { namespace GL { namespace Test { namespace {
@@ -166,7 +167,7 @@ void ContextTest::extensions() {
             #endif
 
             #if defined(MAGNUM_TARGET_GLES) && !defined(MAGNUM_TARGET_GLES2)
-            if(e.coreVersion() == Version::GLES300) {
+            if(e.coreVersion() == Version::GLES300 && e.index() != Extensions::MAGNUM::shader_vertex_id::Index) {
                 Error{} << "Extension" << e.string() << "has core version" << e.coreVersion() << "on a GLES3 build -- it shouldn't be present at all";
                 CORRADE_VERIFY(false);
             }

src/Magnum/MeshTools/FullScreenTriangle.cpp

@@ -40,7 +40,10 @@ GL::Mesh fullScreenTriangle(const GL::Version version) {
     mesh.setPrimitive(GL::MeshPrimitive::Triangles)
         .setCount(3);
 
-    if(!GL::Context::current().isExtensionSupported<GL::Extensions::MAGNUM::shader_vertex_id>(version)) {
+    #ifndef MAGNUM_TARGET_GLES2
+    if(!GL::Context::current().isExtensionSupported<GL::Extensions::MAGNUM::shader_vertex_id>(version))
+    #endif
+    {
         constexpr Vector2 triangle[]{
             {-1.0f,  1.0f},
             {-1.0f, -3.0f},
@@ -51,6 +54,10 @@ GL::Mesh fullScreenTriangle(const GL::Version version) {
         mesh.addVertexBuffer(std::move(buffer), 0, GL::Attribute<0, Vector2>{});
     }
 
+    #ifdef MAGNUM_TARGET_GLES2
+    static_cast<void>(version);
+    #endif
+
     return mesh;
 }
 

src/Magnum/TextureTools/DistanceField.cpp

@@ -106,7 +106,10 @@ DistanceFieldShader::DistanceFieldShader(const UnsignedInt radius) {
 
     attachShaders({vert, frag});
 
-    if(!GL::Context::current().isExtensionSupported<GL::Extensions::MAGNUM::shader_vertex_id>()) {
+    #ifndef MAGNUM_TARGET_GLES2
+    if(!GL::Context::current().isExtensionSupported<GL::Extensions::MAGNUM::shader_vertex_id>())
+    #endif
+    {
         bindAttributeLocation(Position::Location, "position");
     }
 
@@ -154,7 +157,9 @@ DistanceField::DistanceField(const UnsignedInt radius): _state{new State{radius}
     _state->mesh.setPrimitive(GL::MeshPrimitive::Triangles)
         .setCount(3);
 
+    #ifndef MAGNUM_TARGET_GLES2
     if(!GL::Context::current().isExtensionSupported<GL::Extensions::MAGNUM::shader_vertex_id>())
+    #endif
     {
         constexpr Vector2 triangle[] = {
             Vector2(-1.0,  1.0),