gl/vertex_attrib: Fix types

! GL_INT is int32_t and not just int
+ Add GL_UNSIGNED_INT and GL_UNSIGNED_INT64_ARB
! Use glVertexAttribLPointer for long types (double and uint64_t)
! Use glVertexAttribIPointer for integer types
gl: Always use GLEW

! As MESA still doesn't have bindless textures, it's missing the
  required functions (e.g. glGetTextureHandleARB()) as well. Working
  with glext prototypes will therefore fail on linking; so there's no
  choice but to always use GLEW.