* 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

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

* 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 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.

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

This was working in 2.6 but no longer does in current git tree.
It appears touch position is only tracked on
AMOTION_EVENT_ACTION_[DOWN|UP], which only registers the initial touch
on the screen. Subsequent movement is not tracked into CORE.

Touch position and the Gesture System appears to be updated twice in
AndroidInputCallback in what looks like perhaps a copy paste error (code
is identical) with the exception of tracking AMOTION_EVENT_ACTION_UP in
the 2nd copy of the code (but this is not necessary to track).

If you need to track the first touch or release touch position, you can
do so with checking IsMouseButton[Pressed|Released] on the same frame.

This patch makes it so the touch position is always updated, and merges the
duplicated code into 1 singular code path.

NOTE: Two libraries still use custom allocators: glfw and stb_vorbis

NOTE: This functionality has an important issue, line space is fixed to 1.5 font.baseSize pixels, depending on the font and how it has been generated that spacing could be too wide or too narrow...

In `InitGraphicsDevice(...)`, the Android section has a screen security check
like other platforms- but CORE.display.width, CORE.display.height are
not set yet, so the security check sets it to 0. So ensure we query the
device's screen width and height before the screen size security check.

This also gives you the ability to run a proper full-screen application
on Android without any scaling and guess work on the target device by
setting screen width and height to 0 in `InitWindow(...)` and using
`GetScreen[Width|Height]()` to get the actual values.

Improved file access checks

Very weird... lately icons are not properly displayed on browser!

After several reviews, now it seems everything works as expected.