gpu/shaders: use correct type for integer vector

OpenGL supports casting from int to float during vertex array
reading. Direct3D doesn't. Since we're transpiling from GLSL, we can't
directly use the Direct3D builtin "asint". So that leaves using
"ivec2" instead of vec2.

Signed-off-by: Elias Naur <mail@eliasnaur.com>

gpu/backend/backend.go

@@ -166,6 +166,7 @@ const (
 
 const (
 	DataTypeFloat DataType = iota
+	DataTypeInt
 	DataTypeShort
 )