GL: skip also dispatchCompute() if count is zero in any dimension.

To be consistent with what draw() does.

src/Magnum/GL/AbstractShaderProgram.cpp

@@ -519,6 +519,9 @@ AbstractShaderProgram& AbstractShaderProgram::drawTransformFeedback(MeshView& me
 
 #if !defined(MAGNUM_TARGET_GLES2) && !defined(MAGNUM_TARGET_WEBGL)
 AbstractShaderProgram& AbstractShaderProgram::dispatchCompute(const Vector3ui& workgroupCount) {
+    /* Nothing to dispatch, exit without touching any state */
+    if(!workgroupCount.product()) return *this;
+
     use();
     glDispatchCompute(workgroupCount.x(), workgroupCount.y(), workgroupCount.z());
     return *this;

src/Magnum/GL/AbstractShaderProgram.h

@@ -1322,7 +1322,9 @@ class MAGNUM_GL_EXPORT AbstractShaderProgram: public AbstractObject {
          * @param workgroupCount    Workgroup count in given dimension
          * @return Reference to self (for method chaining)
          *
-         * Valid only on programs with compute shader attached.
+         * Valid only on programs with compute shader attached. If
+         * @p workgroupCount is @cpp 0 @ce in any dimension, no compute
+         * dispatch commands are issued.
          * @see @fn_gl{DispatchCompute}
          * @requires_gl43 Extension @gl_extension{ARB,compute_shader}
          * @requires_gles31 Compute shaders are not available in OpenGL ES 3.0