[Impeller] ensure precision matches for buggy vulkan drivers. (#54372)

This is another precision mismatch bug - WHICH IS NOT AN ERROR AT ALL BUT THE PowerVR Drivers ARE BUGGY BUGGY BUGGY

Fixes flutter/flutter#152951

Author: Jonah Williams

impeller/entity/shaders/gradients/fast_gradient.vert

@@ -11,11 +11,11 @@ uniform FrameInfo {
 frame_info;
 
 in vec2 position;
-in vec4 color;
+in mediump vec4 color;
 
 // The geometry of the fast gradient draws is designed so that the
 // varying unit will perform the correct color interpolation.
-out vec4 v_color;
+out mediump vec4 v_color;
 
 void main() {
   gl_Position = frame_info.mvp * vec4(position, 0.0, 1.0);

impeller/tools/malioc.json

@@ -793,7 +793,7 @@
           "stack_spill_bytes": 0,
           "thread_occupancy": 100,
           "uniform_registers_used": 20,
-          "work_registers_used": 9
+          "work_registers_used": 7
         }
       }
     }
@@ -2175,7 +2175,7 @@
           "stack_spill_bytes": 0,
           "thread_occupancy": 100,
           "uniform_registers_used": 8,
-          "work_registers_used": 9
+          "work_registers_used": 7
         }
       }
     },