Shader: Refine preprocessing on GLES platform

Don't globally set float precision to mediump, only fragment
shaders need that and defining it for vertex shaders causes
tilemap cracks.

Also manually define low precision for variables that hold
color / alpha values.

CMakeLists.txt

@@ -210,6 +210,7 @@ source_group("MKXP Source" FILES ${MAIN_SOURCE} ${MAIN_HEADERS})
 ## Setup embedded source ##
 
 set(EMBEDDED_INPUT
+	shader/common.h
 	shader/transSimple.frag
 	shader/trans.frag
 	shader/hue.frag