Implicit conversion from int to float is not supported by
GLSL ES.
Fixes:
(gnome-shell:8954): Cogl-WARNING **: Shader compilation failed:
1:2: P0004: High precision not supported, instead compiling high precision as medium precision
4:17: S0001: Type mismatch in arithmetic operation between 'int' and 'float'
when one trigger the overview mode on Mali 400 r1p1 GPU.
https://bugzilla.gnome.org/show_bug.cgi?id=745442