internal/Graphics: Fix wrong enums for locations
internal/Graphics: Use explicit attribute locations

Removed attribute locations array from the shadow state, as well as
related enumerations. Any object which had to set/get an attribute
location now uses the explicit locations.
Texture: Fix texture unit warning message

Was warning with wrong requested texture unit
Texture: Update warning message
Texture: Minor formatting change
internal/Graphics: Add ClearErrors internal method

Clears all GL error flags, returns number of flags that were set
Merge branch 'attrib-layout' into develop-0.2.0
Use _ClearErrors where needed