Evas masking: Refactor GL code

Evas masking: Refactor GL code

Use the same method as for map masking:
- Pass absolute geometry of the mask to the push functions,
- Compute absolute position in the shader (like gl_Position)
  and apply scaling factor + offset to sample the mask.

Masking now uses a single vec4 attribute instead of hi-jacking
other vertices.

This way, all masking shaders share the same code and are way
simpler. On the other hand, the vertex shaders have a little bit
more work to do.