renderer/opengl: maybe fix issue with cell bg alignment

By using integers for the fragcoords I may have stepped on an edge case
which causes cell background positions to be shifted by 1 px under some
circumstances. I couldn't reproduce that issue in a VM, so I'm making
this commit for the user who was having the problem to test it.

src/renderer/shaders/glsl/cell_bg.f.glsl

@@ -1,7 +1,7 @@
 #include "common.glsl"
 
 // Position the origin to the upper left
-layout(origin_upper_left, pixel_center_integer) in vec4 gl_FragCoord;
+layout(origin_upper_left) in vec4 gl_FragCoord;
 
 // Must declare this output for some versions of OpenGL.
 layout(location = 0) out vec4 out_FragColor;