Revert "Use glTexImage2D rather than glTexSubImage2D when uploading a…

… full texture"

This was causing some code to break with GL_INVALID_OPERATION under some conditions.
See https://www.opengl.org/wiki/Common_Mistakes#Updating_a_texture

This reverts commit 74becf7.

include/pangolin/gl/gl.hpp

@@ -194,7 +194,7 @@ inline void GlTexture::Upload(
     GLenum data_format, GLenum data_type
 ) {
     Bind();
-    glTexImage2D(GL_TEXTURE_2D, 0, internal_format, width, height, 0, data_format, data_type, data);
+    glTexSubImage2D(GL_TEXTURE_2D,0,0,0,width,height,data_format,data_type,data);
     CheckGlDieOnError();
 }