Examples: GL2: rename functions to include GL2 in name

Backup polygon mode, draw with filled polygons, restore state, for clients running opengl with glPolygonMode. closes #798

IO: io.MousePos needs to be set to ImVec2(-FLT_MAX,-FLT_MAX) when mouse is unavailable/missing. Previously ImVec2(-1,-1) was enough but we'll now accept negative mouse coordinates.

Examples: OpenGL2+GLFW/SDL: Added commented out glUseProgram(0) in main.cpp for visibility (#1116)

Comments to redirect people to OpenGL3 examples (#1116)

Fixed overzealous GCC warnings (#883) + partly undo 8553bab

Minor const fixes for overzealous warnings (#883)

Added a void* user_data parameter to Clipboard function handlers. (#875)

Examples: OpenGL2: Uploading font texture as RGBA32 to increase compatibility with users shaders for beginners (#824)

Examples: OpenGL*: Savnig/restoring existing scissor rectangle for completeness (#807)

Examples: Accessing ImVector fields directly, feel less stl-ey and fix renderer side assert when render list only contains a callback (#810)

Examples: Renaming opengl_example/ to opengl2_example/ for clarity (1/2 because git)