Avoid unnecessarily re-allocating texture buffer when the size hasn't changed.
v4:
Incorporated krh and anderco's comments. Now adding newly allocated
buffer's dimensions to texture_damage
v3:
* Removed unnecessary parentheses
* Added check for switching from EGL image to SHM buffer
* Moved shader assignment out of IF condition
v2:
Fixed the wrong comparison
v1:
Depending on specific DRI driver implementation, glTexImage2D() with data
set to NULL may or may not re-allocate the texture buffer each time it is
called. Unintended consequences happen if later glTexSubImage2D() is called
to only update a sub-region of the texture buffer.
I've explored moving glTexImage2D() from gl_renderer_attach() and simply
mark the texture dirty, but the current implemention seems cleaner because
I won't have to worry about calling ensure_textures() and re-assigning
gs->shader unnecessarily.