GL: make the double precision shader tests work on llvmpipe.

It doesn't support GLSL 4.00 yet, but the extension alone is supported.

src/Magnum/GL/Test/AbstractShaderProgramGLTest.cpp

@@ -558,8 +558,8 @@ struct MyDoubleShader: AbstractShaderProgram {
 MyDoubleShader::MyDoubleShader() {
     Utility::Resource rs("AbstractShaderProgramGLTest");
 
-    Shader vert(Version::GL400, Shader::Type::Vertex);
-    Shader frag(Version::GL400, Shader::Type::Fragment);
+    Shader vert(Version::GL320, Shader::Type::Vertex);
+    Shader frag(Version::GL320, Shader::Type::Fragment);
     vert.addSource(rs.get("MyDoubleShader.vert"));
     frag.addSource(rs.get("MyDoubleShader.frag"));
 

src/Magnum/GL/Test/AbstractShaderProgramGLTestFiles/MyDoubleShader.frag

@@ -1,3 +1,5 @@
+#extension GL_ARB_gpu_shader_fp64: require
+
 uniform double multiplier;
 uniform dvec4 color;
 uniform dvec4 additions[3];

src/Magnum/GL/Test/AbstractShaderProgramGLTestFiles/MyDoubleShader.vert

@@ -1,3 +1,5 @@
+#extension GL_ARB_gpu_shader_fp64: require
+
 in vec4 position;
 
 uniform dmat4 matrix;