Test: don't require GLSL 4.10 if not strictly needed.

Fixes test failures on soft Mesa drivers that support only GLSL 3.30.

src/Magnum/Test/MeshGLTest.cpp

@@ -476,10 +476,19 @@ IntegerShader::IntegerShader(const std::string& type) {
 
 #ifndef MAGNUM_TARGET_GLES
 DoubleShader::DoubleShader(const std::string& type, const std::string& outputType, const std::string& conversion) {
-    Shader vert(Version::GL410, Shader::Type::Vertex);
+    constexpr const Version version =
-    Shader frag(Version::GL410, Shader::Type::Fragment);
+        #ifndef CORRADE_TARGET_APPLE
+        Version::GL300;
+        #else
+        Version::GL400;
+        #endif
+    Shader vert{version, Shader::Type::Vertex};
+    Shader frag(version, Shader::Type::Fragment);
 
-    vert.addSource("in " + type + " value;\n"
+    vert.addSource(
+        "#extension GL_ARB_vertex_attrib_64bit: require\n"
+        "#extension GL_ARB_gpu_shader_fp64: require\n"
+        "in " + type + " value;\n"
         "out " + outputType + " valueInterpolated;\n"
         "void main() {\n"
         "    valueInterpolated = " + conversion + ";\n"