mesa: fix validation of GL_RED/GL_RG with float formats for GLES.
- GL_HALF_FLOAT_OES is defined by OES_texture_half_float, not GLES 3.0
- GL_HALF_FLOAT is not supported in GLES 2.0
- when type is GL_FLOAT or GL_HALF_FLOAT, only sized internal formats is allowed in GLES 3.0