GL: fix AbstractShaderProgram compute test on ES 3.1 contexts.

src/Magnum/GL/Test/AbstractShaderProgramGLTestFiles/ComputeShader.comp

@@ -1,6 +1,6 @@
 layout(local_size_x = 2, local_size_y = 2) in;
-layout(binding = 0, rgba8ui) uniform mediump uimage2D inData;
-layout(binding = 1, rgba8ui) uniform mediump uimage2D outData;
+layout(binding = 0, rgba8ui) readonly uniform mediump uimage2D inData;
+layout(binding = 1, rgba8ui) writeonly uniform mediump uimage2D outData;
 
 void main() {
     imageStore(outData, ivec2(gl_GlobalInvocationID.xy), imageLoad(inData, ivec2(gl_GlobalInvocationID.xy))*3u/2u);