Flip image transformation so that image is not mirrored about horizontal line bisecting image.
Use glFragToTex transformation matrix to project image view widget coordinates into image texture space in order to compute the coordinate of the image pixel under the mouse pointer, replacing ad hoc method that was broken by move from vertex shader interpolated texture coordinates to derivation from gl_FragCoord.
Transfer reponsibility of current pointer coordinate & intensity notification and caching from ImageWidget to Renderer.