* Update compilation warning flags

Removed the -Wall flag that allow many warnings and added manually all the warning flags that don't generate warnings in the src/external folder.
Specifically, these are some of the flags reporting errors in src/external:

-Wmissing-prototypes
-Wunused-variable
-Wunused-value
-Wunused-parameter
-Wunused-function

* Uptade compilation flags in examples with missing -std=c99

NB glsl100 shader needs testing on "bare metal"

Co-authored-by: codifies <nospam@antispam.com>

It seems something is not working properly...

I don't like this solution but well... let's use it for the moment...

Co-authored-by: codifies <nospam@antispam.com>

Virtual mouse with window letterbox, and Clamp

* Android: Better track touch input returned from IsMouse*()

Switch to actually tracking touch input to use for "mouse" input rather
than the gestures system. The gesture system as an abstraction ontop of
raw touch input loses some information needed to map to "mouse"
input.

Before,
- IsMouseButtonReleased() triggers immediately after the initial touch
(because GESTURE_TAP activates immediately on touch) instead of waiting for the
touch to be released.
- IsMouseButtonUp() returns false, when it should just be the opposite
of IsMouseButtonDown().
- IsMouseButtonDown() returns true only after GESTURE_HOLD (which
activates after some period of time after GESTURE_TAP), when instead it
should just be true whenever there is touch input i.e. gesture !=
GESTURE_NONE or alternatively when any input is received on the screen.

After this PR, touches map closer to mouse input.
- IsMouseButtonReleased() triggers when touch is released (last frame
was touched, this frame not touched).
- IsMouseButtonUp() returns the opposite of IsMouseButtonDown()
- IsMouseButtonDown() is true when
(AMOTION_EVENT_ACTION_DOWN || AMOTION_EVENT_ACTION_MOVE) and false when
(AMOTION_EVENT_ACTION_UP)

* RPI: Include index check for RPI in GetTouchPosition()

We build the library as debug with AppVeyor and package it this way,
which is unfortunate, because on Windows it's linked against debug
variants of the C runtime. Fix this by build RelWithDebInfo instead

Fixes #1128.

We might want to customize this in feature for multi-config builds (e.g.
a Visual studio build with both Debug and Release configurations).

Output the variable value for user awareness.

head can't think of any improvement I could use on the shaders for
version 120
This has been tested on Linux (Desktop) requires testing on a Rpi

Co-authored-by: codifies <nospam@antispam.com>

Added draw functions:
ImageClearBackground()
ImageDrawPixel()
ImageDrawLineEx()
ImageDrawCircle()