Moved outside BeginDrawing()/EndDrawing() to illustrate drawing is happening to an external texture (not screen)

Some enums values have been renamed to be more consistent and also provide a more detailed description:
 - ShaderLocationIndex:    LOC_VERTEX_POSITION -> SHADER_SHADER_LOC_VERTEX_POSITION
 - ShaderUniformDataType:  UNIFORM_VEC2 -> SHADER_UNIFORM_VEC2
 - MaterialMapType: MAP_ALBEDO -> MATERIAL_MAP_ALBEDO
 - PixelFormat: UNCOMPRESSED_GRAYSCALE -> PIXELFORMAT_UNCOMPRESSED_GRAYSCALE

This function was renamed for consistency in raylib 3.0, just unified all examples to use TextFormat() instead of FormatText()

Now `BeginTextureMode()` does not clean buffers automatically, that behaviour has been changed because there could be some case (i.e. painting software) where texture buffer does not ned to be cleared; user is responsible of clearing buffers using `ClearBackground()`

- Renamed Begin3dMode() --> BeginMode3D()
- Renamed Begin2dMode() --> BeginMode2D()
- Renamed End3dMode() --> EndMode3D()
- Renamed End2dMode() --> EndMode2D()

It's more educational to go through new material system, so, I decide to
remove this function to avoid students confusion...

Functions renamed to improve library consistency

Support render texture size different than screen size