vulkan: Get rid of color in push constants

The color comes in instance data now, no need to abuse push constants
for it.

gsk/resources/vulkan/color.frag.glsl

@@ -2,11 +2,6 @@
 
 layout(location = 0) in vec4 inColor;
 
-layout(push_constant) uniform PushConstants {
-    mat4 mvp;
-    vec4 color;
-} push;
-
 layout(location = 0) out vec4 color;
 
 void main()