Make use of (and require) OpenGL 4.x features present in mesa 11.0+

shaders/object.frag

@@ -1,6 +1,9 @@
 #version 330 core
 #extension GL_ARB_explicit_uniform_location : enable
+#extension GL_ARB_shading_language_420pack : enable
+#ifdef GL_ARB_conservative_depth
 #extension GL_ARB_conservative_depth : enable
+#endif
 
 layout(location = 2) uniform sampler2D texBase;
 layout(location = 3) uniform sampler2DArray texAnim;
@@ -11,7 +14,9 @@ in vec2 fragTC;
 flat in uint fragUseAnimTex;
 
 layout(location = 0) out vec4 color;
+#ifdef GL_ARB_conservative_depth
 layout(depth_unchanged) out float gl_FragDepth;
+#endif
 
 void main(void) {
   vec4 texel = bool(fragUseAnimTex)?texture(texAnim, vec3(fragTC, animFrame)):texture(texBase, fragTC);