Shaders: bind an attribute even if not (dynamically) used on WebGL.

Just alias it to some other attribute. Fixes another WebGL error.

src/Magnum/Shaders/Test/FlatGLTest.cpp

@@ -586,6 +586,12 @@ const struct {
         }}, false, true, false,
         "skinning.tga"},
     {"only secondary set, dynamic", 5, 4, 4, 0, 3, FlatGL2D::Flag::DynamicPerVertexJointCount, {InPlaceInit, {
+            #ifdef MAGNUM_TARGET_WEBGL
+            /* On WebGL the primary joint vertex attribute has to be bound to
+               something even if not (dynamically) used in the end, otherwise
+               it causes an error. So just alias it with the secondary one. */
+            {0, FlatGL2D::JointIds{FlatGL2D::JointIds::Components::Three}},
+            #endif
             {0, FlatGL2D::SecondaryJointIds{FlatGL2D::SecondaryJointIds::Components::Three}},
             {3*4, FlatGL2D::SecondaryWeights{FlatGL2D::SecondaryWeights::Components::Three}},
         }}, true, true, false,

src/Magnum/Shaders/Test/MeshVisualizerGLTest.cpp

@@ -991,6 +991,12 @@ const struct {
     {"only secondary set, dynamic", 5, 4, 4, 0, 3,
         MeshVisualizerGL2D::Flag::DynamicPerVertexJointCount,
         MeshVisualizerGL3D::Flag::DynamicPerVertexJointCount, {InPlaceInit, {
+            #ifdef MAGNUM_TARGET_WEBGL
+            /* On WebGL the primary joint vertex attribute has to be bound to
+               something even if not (dynamically) used in the end, otherwise
+               it causes an error. So just alias it with the secondary one. */
+            {0, MeshVisualizerGL2D::JointIds{MeshVisualizerGL2D::JointIds::Components::Three}},
+            #endif
             {0, MeshVisualizerGL2D::SecondaryJointIds{MeshVisualizerGL2D::SecondaryJointIds::Components::Three}},
             {3*4, MeshVisualizerGL2D::SecondaryWeights{MeshVisualizerGL2D::SecondaryWeights::Components::Three}},
         }}, true, true, false,

src/Magnum/Shaders/Test/PhongGLTest.cpp

@@ -901,6 +901,12 @@ const struct {
         }}, false, true, false,
         "skinning.tga"},
     {"only secondary set, dynamic", 5, 4, 4, 0, 3, PhongGL::Flag::DynamicPerVertexJointCount, {InPlaceInit, {
+            #ifdef MAGNUM_TARGET_WEBGL
+            /* On WebGL the primary joint vertex attribute has to be bound to
+               something even if not (dynamically) used in the end, otherwise
+               it causes an error. So just alias it with the secondary one. */
+            {0, PhongGL::JointIds{PhongGL::JointIds::Components::Three}},
+            #endif
             {0, PhongGL::SecondaryJointIds{PhongGL::SecondaryJointIds::Components::Three}},
             {3*4, PhongGL::SecondaryWeights{PhongGL::SecondaryWeights::Components::Three}},
         }}, true, true, false,