Screen/Icon: select the texture shader
OpenGL/Canvas: remove unused methods CopyNotOr(), CopyAnd()
OpenGL/Canvas: use invert_shader in StretchNot()
OpenGL/Canvas: fix black text color

Regression by commit 8cfe3228
OpenGL/Canvas: reimplement StretchNot() using GL_COMBINE

GL_COMBINE is faster than GL_COLOR_LOGIC_OP.
OpenGL/Scope: remove unused class GLLogicOp
OpenGL/Init: remove Vivante OES_draw_texture workaround

Now that we've eliminated the last user of GL_COLOR_LOGIC_OP, we can
safely use OES_draw_texture on Vivante GPUs.